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.
I think you need to be a bit more specific. As an agnostic who is very interested in biblical scholarship I would say you would have to know what translation they rate 5 stars. If they rate the New Oxford Annotated Bible that high then it’s not a red flag they probably are just interested in Bible scholarship. I would be careful though if they seem to love the English Standard Version.