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.
Aside from using the exact same twist in each of his Robert Langdon books, the major problem I have is how he presents his content.
I have zero problems with historical fiction if it’s written well. I love the genre and I think it’s a great source of literature. The problem with Dan Brown is that he advertises his fiction as being based in fact and verified research, when in reality, it’s drawn from half truths or just blatantly made up