I would never offer unsolicited advice on a quilt. If asked I will give an honest reply and not try to make the quilter feel better about something they should fix (or practice more on...).
I don't comment on everything I see. If a comment pops into my mind when I see a photo, I do try to post it. There are always some I don't care for so if I have nothing good to say, I say nothing. I don't expect everyone to like my quilts, either.
If someone is a newer quilter and their photo doesn't have many comments, I may try to find something positive to comment on. Sometimes it's not hard at all!