Th Lynch movie had some real issues. I felt though that it at least attempted to retain some of the ambiguities in the book, and the political maneuverings.
Structurally I think the biggest issue with the new films is the almost total lack of the Guild, which drives so much of the plot, albeit behind the scenes. You get a sense of the Bene Gesserit's machinations, but not the Spacing Guild. The end of the second movie in some ways seems strange, and I think it's because in the book, it's the Guild that requires acquiescence to Paul's demands, because they see through their prescience that he could, and would destroy the spice. It's a real existential threat to the whole society and the Guild in particular.
But they did some bizarre things with Jessica - making her vomit! and appear nervous all the time - a trained Bene Gesserit? - they destroyed Stilgar's character by making him a bufoon, and pandered to the audience in making the distinction between Fremen "fundamentalists" and the rest, and generally making their society fall in line with modern attitudes. Which I find weird because it's not like the Fremen as written were in line with modern attitudes when the books were came out - they were a harsh, alien culture. And they, of course, turned Chani into a girlboss, because what she was wasn't enough, apparently, and the audience are too dumb to understand why anyone would accept political marriages are a thing.
Anyway - I could rant on for a long time, as it's my favorite book series, so I'll stop there!