Dune Messiah has a lot going for it, like the inner workings of palace intrigue and some fantastic elements of world-building (a prime example being the Tleilaxu and their biological tech).
There’s also Irulan becoming a key character, her Bene Gesserit POV has a ton and a half of cinematic potential, especially with Florence Pugh at the helm.

But how do you make that Paul and Chani arc satisfying for broad audiences? Also and maybe particularly for reader fans, with how much Denis changed Chani’s character from the first book. And even though Paul is galactic emperor, the scale of the story and events does feel smaller than in the first book, there are no new true paradigm shifts.

Things don’t get swashbuckling and truly epic again until Children Of Dune, which I loved.