Before I became a copyeditor, I would review books and would leave an honest, sometimes scathing, review of books that were genuinely bad. I always tried to mention specifics, though, so I didn't get lumped into the "hated it because the packaging was damaged" crowd. However, after editing dozens of manuscripts over the years, I understand the need to show kindness even while telling the truth about what's wrong with a book. Authors are real people, and as you mentioned, the majority are not rich and famous from their books. They're trying—most of them, anyway—to put out the best product they're capable of doing. I now try to review in a way that pictures me speaking to them face to face, and that tempers a lot of the snark, I've found. The ones who don't read the room? Well, they'll eventually get the picture when their book sales come to a halt.