I think there are a few reasons for people being hard to type, one of which is neurosis/psychosis. Some argue that it is easier to type someone with a personality disorder because certain types are inclined to certain disorders. I do think this CAN be true... however...
Even there's the discorder that "belongs" to each type, any type can have any disorder. When people are hard to type, it's because they have been molded by their environment to be something different. There are chemical issues too... like low dopaimine or seratonin, which will cause certain disorders in people regardless of type... I, for example, thought I was either INTp, ISFp, ISTj a few years back when I was still really depressed (looking at the corresponding Oldham types that is).
Not that I'm implying that all people who are hard to type had disorders. What I'm getting at is that when someone can't figure out they're type it's because their environment rewarded a certain behavior which was not one of their first two functions. I think this can lead to a lot of frustration, a low self of self worth, and (at the VERY least) confusion about one's true identity.
I also like my graph idea... picture a graph. At the left and right are the two subtypes and at the top is mental wellness and at the bottom is the type's corresponding personality disorder. I was thinking though that maybe the top should be self-actualization since the middle of the graph should be "normal" rather than "halfway psychotic".