As a counterexample, I have self-studied queuing theory, extreme value analysis, combinatorics, generalised linear regression, etc. and had no problem with it.

Category theory I just can't get through. Either it does not fit with my brain or I fail to see the practical implications, but something makes it not stick.

(Curiously, differential equations (beyond the basics) are in a similar spot for me, even though they are obviously immediately practical in a huge number of areas!)