Here, late, via metafandom and wanting to comment on this because it's such an interesting topic.
My main fandom is Silmarillion and the character I write most, Maedhros, is disabled in canon - an injury where he lost his right hand. A few of the stories I have written have focussed on his actual disability - once I wrote about what happened when he injured his good arm and was (albeit temporarily) completely helpless, and how much he hated that.
I have also written original fic in which the main POV character was blind. That was the hardest fic I've ever written - having to concentrate ALL the time, on not describing anything visually.
I've only once written fic in which a character not already disabled in canon becomes disabled in my fic. That felt just - wrong - to me, and I've never attempted it again.
I think its hard to write characters, especially main characters, that are cognitively disabled, because by the nature of their disability they're not able to contribute to the actual story very much. Having said that I have read several Silmarillion fics in which Maglor (Maedhros's younger brother) is actually autistic, and that worked very well. I have written fic involving characters dealing with issues like clinical depression, alcoholism, bipolar disorder. While they are clearly dealing with a chronic and long term/permanent condition, its debatable how much these things count as an actual disability.