I know I personally have both received concrit on AO3 and appreciated it and given it and had it appreciated. I think it just depends on your tone and what the criticism is. For example, if you see a couple spelling or grammar errors or stuff that needs Brit or America picking, that's probably going to go over okay. Essentially offering enough concrit that you're rewriting or beta-ing the story without being asked? Yeah, don't do that.

I will say as an author, I always prefer something constructive and specific (good or bad) as a comment over just a "good job" type of comment.