There is a lot of criticism these days about biased book reviews and writers reviewing each others books in order to post glowing reviews. I write reviews for Suspense Magazine, and I also occasionally read a book by a fellow writer and post the review on Amazon and GoodReads. I never write a negative review. And I have read some books that I did not enjoy at all.
Even if I don't like a book, there is probably someone out there who will. There is no reason to trash someone hard work. I describe the plot and characters in the novel, and try to place to story in a category. If it is a story of zombie aliens rampaging the countryside, I make that clear. It's not a story that appeals to me, but it appeals to some readers.
If I really believe some parts of the book are badly written, I may just question those aspects and try to put it in perspective. If a writer uses an expression that is regional, but doesn't fit the character, I will mention it. If a writer makes a small error, as in a recent example when I reviewed a book in which the "French press was whistling in the kitchen," my antenna for errors goes up, but I won't mention it. Everyone makes mistakes.
I have seen reviews that criticized the use of swear words in a novel, and either too many sex scenes or too few. As a reviewer, I want to be as honest as possible without dwelling on the weaknesses of the book or the author.
As a reader, I want to read reviews that tell me what to expect, but I always take extremely negative reviews (of anything, not just books) with a grain of salt. Long, negative rants just sound like the reviewer had a bad day, or dislikes the book or author for some other reason.