I’ll go with the low-hanging fruit: Mein Kampf. I’ve read it, cover to cover. As a piece of propaganda, it’s good. As an example of good writing? Absolutely not (though I will admit I have only read it in translation). Oh, and the whole fascist, racist, and generally shitty worldview of the author that he infuses into the text. And the fact that the author is literally Hitler. You 5-star that book? You’re a Nazi. Period. And as a Jewish person, I don’t look too kindly on them.
Well, there’s also the difference in how many people interpret five stars. Some people think that a book has to be one of the best ever, in order to get five stars. Other people think that the books has to have something objectively wrong, in order to get less than five stars. Lots of reasonable, likeable people will give 5 stars to books that were nothing special, but they had a forgettable good time with.