I read the first book between Part I and II and I discovered that I'd forgotten how important the fanaticism was to the plot. Paul spends a significant amount of time and energy trying to avert the inevitable jihad.
That said, there was a lot less open worship of Paul in the book. In the movie there's so much rapturous "Lisan al-Gaib!" that it's become a meme.
I don't mind this change, but it is a change. I do wonder what the implications are for Part III, since I haven't read Messiah yet.
Villeneuve is a brilliant cinematographer. Every frame of the movie could be printed and mounted on a wall. Blade Runner 2049, Arrival, and Sicario were likewise.
If you want to see a jarring directorial change, watch the Villeneuve-directed Sicario and then watch its not-Villeneuve-directed sequel.