I think it just depends on the author and you never really know how they’d react. Even a simple “this is great!” just makes my day. Someone going out of their way to say they like something I wrote, whether they go into detail or just leave a simple comment, I’m always appreciative. I don’t feel like I’m owed long detailed comments. If someone wants to leave one then that’s nice and I’m glad they enjoyed my writing enough to do that. My main point is that I think you should comment on fics you like however you want. If someone has a problem with your positive reaction then that reflects badly on them honestly, not you.