As a writer and former editor, I could critique any book I read. I stopped reading fiction for several years, after I stopped editing, because I couldn't just enjoy the book. Now, while I await the start of several events, beginning with Nanowrimo Camp, I'm reading a great deal, average four books a day.
The only judgment I make is if I liked the book. You won't learn if I think a book has big problems. The author will. Only if that author responds with an attack, not if the advice isn't taken, will I post a negative review. It's a compromise, but as fair as I can make it for both readers and my colleagues. I make only positive recommendations, in public.