My experience with writers of fiction (and I'll limit myself to talking about fiction here, as I don't think I've ever reviewed non-fiction) is that they are generally genuinely doing their best--and also that if a writer is far enough from having a skill, even very clear comments might not get them to grok whatever it is you're saying. Now, this is fine in a writer's group, where they might sleep on it and wake up with a satori, or at least come to understand the issue and fix it one way or another in the months before publication.

But when it comes to already-published books, I have come to realize that I don't want to publicly review books unless, overall, I loved them. It's too late for fixing things, after all, and someone poured many hours of their life into this creative endeavor. Creative work does not have to be perfect to have value (and as a writer and musician I say thank goodness for that). But I don't have to spend more of my time on a thing I didn't like--and that, unlike a textbook, is very unlikely to be revised at any time in the future.

Before publication is a different matter altogether. As a creator, I am grateful for beta readers for helping me make a story better before an editor sees it, and grateful for editors who hopefully will have the time and insight to help me make a story better before the general public sees it. And I'm willing to do the same for an unpublished work, though I generally think of that as a "critique" rather than a "review".