I do have a few cognitive issues (ME, chemo, not enough sleep etc causing brain fog and falling asleep as I'm trying to read), but they usually don't affect my reading comprehension, or not much.
But I have occasionally found a post on here that I had real trouble understanding (not just here of course, I sometimes find posts like this on other sites as well).
I think it may be partly spelling and grammatical issues, but also a tendency to post in a run-on stream of consciousness style, with sentence fragments and jumping from idea to idea in a way that seems incoherent. The post might have no paragraph breaks, but sometimes every sentence has its own line. That kind of post is pretty rare but it does sometimes happen. I'll try to read it, and sometimes with enough concentration it does begin to make some sense, but sometimes I have to give it up and go on to something else.
I don't comment, though, for the reasons you mention. I don't know if other people have cognitive issues, dyslexia or something else and are finding it hard. I mean I sometimes struggle myself to write coherently, so I should understand. And I am absolutely one of those annoying pedantic fuckers so I have to mentally sit on my hands not to correct things sometimes but so far with success! 😁