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.
Just means you’re not the target audience IMO. YA tends to be pacier and some adults still like reading coming of age stories. Some YA is absolutely beautifully written, just like some adult books aren’t well written. I read both and enjoy both. Younger doesn’t mean simplistic.