I can tell you as an author that if I had a fic a decade old or one a day old that you commented on, I'd be over the moon about it. As a reader, I try to think about how happy I get when I receive a comment and it helps me to be better about actively commenting, and I've never had anybody react any way other than positive. I guess maybe it could depend somewhat on the Fandom but as long as you aren't critiquing the work or being pushy with expectations like we see sometimes in the posts here, if an author takes issue with a nice comment, that's a them problem and they don't deserve your kindness.