“It is almost impossible to mention books in bulk without grossly overpraising the great majority of them. Until one has some kind of professional relationship with books one does not discover how bad the majority of them are . . . . But the public will not pay to read that kind of thing.”
That was George Orwell, writing in 1946.
I will add: since you’re going to praise a book that you may not think that great, betraying your inner call to truth and beauty, you might as well carefully pick the book you’re going the praise.
You might as well go for a book written by somebody who has helped you or will be able to help you in the future, or that of a friend, right? It’s only human nature. That’s why I like commenting on old-ish books, or those by people who, chances are, will never have the chance to repay the compliment/stab in the back.