I don't know if Fi egos are are overall more easily offended in terms of quantity/frequency of offense but I think a few things contribute to this perception: 1) their buttons are sometimes idiosyncratic and it can be difficult to predict what will offend them unless you know them well, 2) they're confident in their perception that you said/did something 'wrong' even if it isn't globally perceived as a wrong, and 3) they're constantly evaluating how they stand in relation to you and how subjectively good/bad they feel you are, so if they're offended it carries a certain weight with respect to your relationship (this is probably true for anyone but it's more at the forefront with them). These things can be nerve racking to deal with.