There is something inherently insidious in any typology with more than 4 categories. It's largely something that spontaneously arises too, and it is created by the fact that typology is not values-free, which should make it even more alarming.

What I'm referring to doesn't happen in typologies with 4 or less categories and why it doesn't is simpler than directly explaining all the reasons it happens in larger ones. Take Hogwarts houses. This isn't a serious psychological typology or supposed to be innate at all (you get to choose your house) but a lot of people have commented that it basically shows psychological traits. You can basically divide it up into two axes, a light and warm vs. dark and cold axis (Gryffindor and Hufflepuff vs. Ravenclaw and Slytherin) and an active vs. passive axis (Gryffindor and Slytherin vs. Ravenclaw and Hufflepuff), and then there's an axis for houses being favored or unfavored in the books (Gryffindor and Ravenclaw vs. Slytherin and Hufflepuff). If the majority of people favor one of either axis, more people will shift to the other axis to be edgy or hipsters or just express alienation and dissatisfaction in general. That shifts the other dimensional axis to being the dominant one, so people will start shifting to a house there with the same trait on the other axis. If everyone wants to be in Gryffindor and Slytherin to be relevant, people from there will switch to nearby Hufflepuff and Ravenclaw. If more people are in Gryffindor and Hufflepuff to be light, they'll start switching into Slytherin and Ravenclaw. The mutability of the houses is created by the obvious perceptibility of the various traits of all of the houses. You see the same thing with the four temperaments, introverts and extraverts, elements, etc.

On the other hand, any system with more than four types either has obvious asymmetries, or too many types to be obviously perceptible (usually these play off of each other, since the symmetries add order to the system and if it's just a complete mess, it doesn't matter how mathematically intricate your system is. Systems with five types tend to either over-value or under-value one of the types compared to the other five, systems with six introduce height- and direction-metaphors that imply hierarchy based on the sheer fact of how they have to be represented physically, systems with seven have the same issue as with five, and systems with more than that can't be remembered without first learning about the types and forming prejudices that become self-sustaining, due to people not being able to remember more than 7 items of information regarding an unfamiliar topic). All the insidiousness arises out of the dynamics of a larger system just because of their nature and here's how it works in action.

First, socionics. Socionics is supposed to treat all of the types as equal but different. Aside from separate but equal not really being a thing, which types get chosen to be valued in socionics is not even vaguely random. The valued types tend to be largely quadra-based due to the tribalism that arises through socionics's four quadra system (which is basically a sub-typology like Hogwarts houses that makes things much more interesting than MBTI), but the most valued types (on this forum) are IEI and SLE, and the most valued quadra is Beta Quadra. What makes the system really insidious rather than just eyeroll-worthy is the existence of dreck types. The dreck types in any system always outnumber the valued types, like dreck always outnumbers what's well-made. In socionics the dreck types are ESE, ILE, LII, and IEE, and Alpha Quadra is consigned to being a dreck quadra due to the presence of 3/4 of the dreck types (SEI is not a dreck type due to the overall dynamic of the system, which is why it's basically completely uncontroversial to self-type as SEI). IEI is the most valuable type simply due to the nature of socionics typology. It tends to be used as a way for people to know themselves and to improve. Deep knowledge is associated with Ni in socionics, and personal and emotional awareness is associated with Ethics > Logics, since NTs are generally portrayed as robots who want to be deep and emotional at best, like some sort of Pinocchio wanting to be a real boy. The Te PoLR in IEI is a bonus, because it basically describes IEIs as sucking at anything practical or logical in the common interpretation. People sucking at anything is because they're too good for it, essentially. SLE is valued due to being the other half of IEI, someone who can do everything with utter ease and who doesn't think deeply because they don't have to, like a rampant medieval saga hero but in real life, and who IEIs can acceptably envy due to the theory of duality applying to all the types, without them (or SLEs) having to give up their status at the top due to not being good at something. Beta is chosen on this forum due to containing those types and due to people like mu4 who the overwhelming majority of people are irritated with self-typing in Gamma, Alpha being the dreck quadra everywhere, and Delta being diametrically opposed to Beta while not containing as good or popular types.

At this point, everything I've described still just sounds like a nuisance, but here's where you get the insidious part of it all: Ne is considered shallow, ADHD, childish, a spaz, and dreck (although EII tends to be a moderately-good to very-good-but-not-pinacled type due to being Literally an Empath, having extremely good morality, and in a neutral quadra). This is not in retaliation to the original socionists over-favoring it, or MBTI or Jung over-favoring Ni. Ni was opposed to Si in all of these systems, not Ne. The reason Ne is the target is because perspective-taking, nihilism, and whimsy all tend to be (often incorrectly) attributed to it, and if people used more of this, socionics would simply not be able to maintain itself the way it does on this forum. It's like finches evolving longer beaks if they have to peck into weird trees in the Galapagos, not anything anyone designed. If anyone thought it might be possible that socionics wasn't the Ultimate Truth of Reality, they'd go look at other things, get a life, and decide that socionics is not really worth their time, so anyone looking at other possibilities must be doing it because they are shallow. If people decide to devalue socionics (a nihilistic approach to the theory) or do it just for fun, it must be because they are childish and not serious or deep in general (never mind that just the word "nihilism" tends to imply "3edgy5me" and "3deep5me" and therefore Ni, nihilism is expressed through farce and contradiction so people see it as Ne and annoying). It is also very difficult to nearly impossible to re-type from a dreck type to a valued type, since Ne-leads seeing all the different possibilities and ESEs being suggestible that way combined with not being able to see the larger picture are exactly the people who would be confused and retype themselves, while IEIs and other higher-value types would very decisively know who they are and not have to (never mind that the types are projections and the theory is legitimately based on higher-level mathematics and not widely agreed-upon in its concrete manifestations, which makes it much easier to retype in socionics than enneagram, but that barely lowers the disturbing way the theory perpetuates itself). Getting a type in the first place is an investment of putting out (often deeply) personal information that you'd never put anywhere else on the Internet except social media, so once people have a type that they feel is good enough, they will defend it by putting out more personal information when people challenge them with conflicting observations. Since people feel exposed from doing that in the first place, they tend to stick around, which is the investment quality. Deciding typology is nonsense and quitting, or even a fun game worth continuing for fun makes the investment of deeply personal and serious information in it seem worthless and like a threat, since there's no way to take information back and once it's out there, it's out there. So the system is essentially perpetuated through loyalty and in socionics, this has its pinnacle in the IEI type. At the very least, SLE has the excuse of having no internal life if they'd like to fall back on that (although this is a cost in itself, it's essentially a one-time cost), but IEI must constantly renew itself with exposure to prove that it has a deep inner life, which also tends to mean deep exposure, or risk being re-typed into dreck types.

Enneagram actually has the exact same thing going on. In enneagram, the dreck types are 2, 6 and 9, the dreck instinct stacking is So/Sx, and the dreck tritypes are 2-6-9 and 3-6-9. The valued tritypes are 4-5-8 (especially this one, despite the "dark" reputation) and 4-5-9, the valued instinct stackings are Sx/Sp and Sp/Sx, and the valued types are 4 and 8. The valued types being 4 and 8 are basically exactly the same tired tropes as IEI and SLE in socionics, and the devalued types are basically the same issue: people have no real understanding of anything, except in this case, it's due to being too social and conformist rather than to being too scatterbrained and superficial (although notice the similar motif with So/Sx's flightiness). 4 has the interesting added trait of having to be "authentic", "deep", and "self-revealing" that forces people to put massive amounts of just incredibly personal information out there just to get it (unless you do the Real Romantic Cheat like me and just say "I'm too unique and complex for an enneagram type. I don't want to be part of a group anyways". This should be the criteria for being a real 4: being too good for typology, now go your own way and have fun with your poetry and trees. But this does not sustain the system at all since it just drives snobs out and having an inaccessible type only makes people want it and change the criteria, so it gets evolved out naturally). E2 is devalued since 4 disintegrates to 2 and the beaten-up people who identify with E4 put all of the characteristics of their narcissistic parents into E2 ("the image of being helpful, without necessarily being helpful" rather than the real problem actually helpful people have of being too accommodating to the point where they're squashed before they can do anything, which is instead moved to E9 so people don't have to have such a cynical view of humanity). 4-5-8 is the most value type because it is dominant and 4-5-9 is submissive. People typing in 4-5-8 are either not particularly into enneagram communities, or they're the people perpetuating them. The 4-5-9 tritype is "more enlightened and spiritual" but notice that you have to be completely accommodating within the community to be typed as it in general, which means a surrender of the will. This essentially traps you within the community (unless you're using enneagram casually, but most of those people will pick 4-5-1 over 4-5-9). The 4-5-8s have to ramble about how dark and evil they are, but this is not a real downside. Batman is faaaaar more popular than Superman for a reason, and it's the same reason vampires are considered sexy or people like reading The Stranger by Camus. People want an excuse to embrace egotism and their dark side, desperately, and "Man, I just want to strangle him, I'm so angry!" is frankly something that everyone has thought at at least one point even if they don't really mean it. 4-5-8 in enneagram is like the sort of "rebellion" that is actually normal due to a false view of normal being perpetuated by everyone (see: TV talk shows and their "controversial views" that are literally mainstream) but it's still considered a superior type so you have the same problem where people have to invest in it like in socionics (4-5-9s have the same problem too, but they get their type through someone else giving it to them in the first place since no real triple-withdrawn would type completely of their own volition. You need good references to apply to be 4-5-9). Notice that the 4-5-8 descriptions all focus on how they view their world from the inside while the 4-5-9 descriptions are all about how passive and avoidant and innocent they are compared to other people in social situations, like it's some sort of highly-introverted version of a 3-6-9. This is exactly the same dynamic that Chae mentions in her post against BDSM where BDSM communities are a way to perpetuate abuse. People say that 4-5-9s are the most spiritual type to create the illusion that people want to type as that and be enneagram subs/abused, while enneagram doms/abusers are 4-5-8s (this doesn't go for people who aren't deeply involved in the communities and just want an enneagram Pisces or Scorpio-type equivalent, as I said before, and this works the same way that the valuation of types works in the first place: there are too many tritypes for people to value them all without learning the values, so often people independently going through it will often pick 2-6-1, 3-7-8, or some other, generally extreme-sounding, tritype that sounds like what they normally value but that enneagram communities don't even care about one way or another as their personal favorite. Sometimes people will even pick dreck tritypes according to the communities based on their understanding of those tritypes).

In both typologies, you have the claim "people don't really know themselves" as the threat, which is often completely absurd when presented to people who have detailed records of their lives and thoughts in various forms. This is basically my earlier Nietzsche/Romantic point about individualism: if people are their inner selves, what's looking inwards? As Wittgenstein said, there's no private language, so it's the collective that makes language (the Gattung) looking inwards to the individual. So denying that someone knows themselves in a situation where it'd generally be agreed that they know themselves by people who don't have a vested interest (diarists, bloggers, etc. are not usually argued with on this basis, because the content of what they do and think is more important than the fact of the person being introspective since introspection is a value propagated by typology but not really society at large in all contexts) is simply a way of saying "the Gattung disagrees" with people inserting themselves as the authority for the Gattung. It is a power-play that tries to ontologically violate and invalidate people, and it is sustained by sacrifices (preferably other people's). All because people have to perpetually renew their denial that they gave up so much that this society considers of utmost value for nothing.

Hopefully I did good enough explaining that. I don't want to have wasted time explaining what seems like obvious patterns to me but is probably going to get at least some weak roars of "You're so shallow and un-spiritual and missing the typology Gattung's point!" from the audience. These are patterns I've put together over time. You're going to have to argue against my premises or reasoning to dismantle it or you're reinforcing my point.