I think a lot of critters get caught up on their own pet peeves and don't think about what state the current draft is in. There is no point nitpicking grammar if there are big worries about content, characterization, etc. I generally refuse to line edit for free--a real quality line edit takes a LOT of attention. And there is no point line editing a book still undergoing major revisions. It is better to focus on recurring issues (like if someone uses passive voice extensively) than really specific examples. If you can help someone fix their recurring issues earlier and learn what i wrong, there will be less line edits needed later.

But unless they are getting close to final draft state, it is better to focus on the stuff you said. Also things like whether the author misplaces characters, contradicts the characterization, etc.

I sure wish I had the energy and time to get back into more of this stuff right now. I tend to have a knack for it, but if I get too many things to do, I start stressing about doing enough back.