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.
It depends. I love the first book in the Wheel of Time series (I read it so long ago that I see I don’t even remember its name). I read five or six of the sequels, and yeah, they’re problematic in many ways, and also somewhat boring (one of them was a six hundred page book taking place in something like three days or so).
So I guess my point is, if that person has read the start of the series and thought it great is far less of a red flag than him reading most of the series and thinking it great. Although I’d be super impressed with the latter for keeping track of all those characters!