Dune Messiah is much shorter than Dune, at times brisker. But its narrative often meanders in a way Dune's never did. The actors in the conspiracy are at cross-purposes in many ways, which will help Paul outmaneuver them. Yet at times, it's hard to make sense of their actions. The most clear motivations come from the Bene Gesserit, who are determined to recover the genetic lines lost to their selective breeding program when Paul, after becoming the Kwisatz Haderach, immediately became their enemy and took the Fremen girl Chani as his true wife. Paul tries to cut a deal with them (mostly to save Chani's life, as she's in the Bene Gesserit's crosshairs) by which he will agree to bear a child with Irulan, herself a Bene Gesserit, but only through artificial insemination, and the child will not be named heir to the throne. Meanwhile, among the conspirators are another new order, the Bene Tleilax, who have some pretty incredible talents of their own. Their plan involves a being called a ghola, named Hayt, who is in fact the revived Duncan Idaho, killed defending Paul in Dune. There is also the Tleilaxu "Face Dancer" named Scytale, who can alter his appearance to that of anyone, and intends to infiltrate Paul's household in that way.
Whereas Dune dazzled in its range, offering everything from adventure spectacle to commentary on world affairs to family drama, Dune Messiah replaces much of the original's stirring narrative with exposition, as Paul Muad'Dib attempts to fend off a conspiracy against him and hold on to what little is meaningful to him personally in a universe ravaged by war in his name. Having raised Paul Atreides to the greatest heights possible, Herbert now proceeds to tear him down. That ought to be immensely powerful stuff, tragedy on a Shakespearean scale at least. The real tragedy is in how muted this story's impact is.
The law of diminishing returns. Dune was great. Dune Messiah very good but I found they went downhill from there with the Brian Herbert books being virtually unreadable.