I can't really remember of all time, but recently I started reading Dune: Messiah, and I had to stop reading it was so bad. I might be in the minority but the tonal shifts, changes in character attitudes, and jumping right into these assassination plots, all of it just came out weird and misplaced. Definitely did not slap with even 1/4th the power of Dune.
Herbert didn't want to continue Dune and was pressured to write a follow up. It was an era when most science fiction was still published in periodicals. The first half of Messiah are the results that were then compiled into the start. It is like a really shitty draft. Everyone experiences the same thing. I put it down for quite a while too. If you can make it to the second half, it will become one you can't put down, like the first. It does setup well for what is to come. After I got back into Messiah, I read all the way to the end of the entire series, even the Brian Herbert/Kevin Anderson stuff. Those last two are not like Frank's writings, but are their own thing and still more readable than the first half of Messiah. IMO the first half of Messiah is a great example of what happens when Art takes a back seat to an anxious banking type mentality. Bankers make terrible artists and advisors.
GEoD is IMO the best book in the series as it eviscerates many cultural norms and deep assumptions like fascist altruism, eternal boredom, the coexistence of misogyny and feminism, manipulation that is both brutal and kind, and if an alien can be human. It even infers the question of potential delusional prescience in my opinion. It will make you think about the motivation of leaders and what you may endure because of their vision of a future.
Don't read the prequels by Brin and Bear, they are not only awful but also steer the lore into really dumb place which i'm pretty sure was not intended by Asimov. Though to be honest the two prequels by Asimov are also much worse than the main series.
I mean, none of that is true, and Herbert stated he had parts of Messiah and Children written before Dune was even finished.
In the forward to Heretics of Dune: "Parts of Dune Messiah and Children of Dune were written before Dune was completed. They fleshed out more in the writing, but the essential story remained intact."
I read the first four dune books this year and I think they all suffer from the same problem, that is they have interesting characters, original lore, great world building, but nothing interesting happens until the very ending of the book. They all felt like a slog to get through to me.