Agreed. In my early fandom days people would outright state "concrit welcome," which I would say is the only time it's appropriate to offer criticism on the technical aspects of the story. Rarely do I see authors looking for that kind of thing currently.
Fandom is not a creative writing class. This is not a workshop. Fics can be posted in varying degrees of polished/finished draft, from "no beta we die like x" to fics that have been painstakingly edited before posting. Some fics are written to completion before posting, some are posted bit by bit as the author figures the story out.
The role of commenter is far more fan and cheerleader than critic. Positive comments on the technical aspects of a story are always welcome, obviously, but once a fic is posted, it's generally more discouraging and even hurtful to have someone point out what didn't work for them. If what doesn't work grated enough that you feel the urge to comment on it, better to just stop reading.
One exception I can think of is if there's an easily fixed factual error or glaring typo? And even then, I usually state clearly in the notes that I'm open to corrections on that kind of thing (ex: I got the name of the frontman wrong when writing about characters attending a concert, and I was SUPER grateful to have that pointed out).
In general, unless the author has made it clear that they want constructive criticism or corrections, stick to the positive. Asking about an author's thought process behind certain choices is fine, so long as it comes from a place of curiosity rather than complaint.
I'm sure there are authors out there who don't mind critical comments, but in my experience, they're in the minority. If you are one of those authors, make that clear in the notes or tags of your story. Otherwise, readers should assume the default preference is positive/encouraging comments.
