Yup. I started the next book once (Heretics?), but even from the start it just felt like Frank Herbert had locked himself in with these characters and this universe, and no longer had any idea what he wanted to do with it, the story just had to go on regardless.

I think a lot of what makes the first book so good is that there's this whole new and mysterious universe, and he keeps it rather mysterious throughout. You don't really know what the Bene Geserit are, what a Mentat is, or even really understand what spice is. You're stuck on a strange planet in a strange universe, learning how things work as you go. That puts you in Paul's shoes, and makes the book great.

Once all these concepts are established and have lost their novelty, I honestly don't feel like there's much appeal to the storytelling. Throughout God Emperor and the first chapters of Heretics it started feeling like a slog, just based on the same established concepts and characters, with very little real development.