Jihad, Honkai, and the Weight of Prophecy
The later cycle, the books written after Herbert's death by his son and the collaborators who finished them, divide readers into two camps. One camp finds them gripping, and the other finds them a museum of the original with the labels changed. I have moved back and forth and now sit, cautiously, in between.
Their subject is faith used as a political instrument, and they are better at that theme than the originals. The premise of the great war is that the Imperium, in its slow decline, uses a religious conflict on one frontier to keep the cohesion of the realm it is losing everywhere else. That is a precise and unhappy idea. Faith becomes an instrument of state policy because it is the only form of loyalty nobody has to be paid for, and the books are at their sharpest when they show sincere people doing terrible things because the cause is genuine and the method is bureaucratic.
The weight of prophecy is the second theme. In the prequels the plan is old, the plan is known to a few, and the plan is not revisable. Everyone acts inside a shape laid down before they were born, and the tragedy is not that the plan is wrong but that nobody is allowed to say so. There is a particular kind of exhaustion in those pages: the sense of people spending their whole lives inside an instruction from the previous generation.
The third theme is more uncomfortable still. The same organisation that engineers a bloodline across centuries also arranges the erasure of an entire species from collective memory, and it treats that erasure as good practice. Read these books and you will find yourself admiring the competence and disliking the warmth, which is exactly the effect the Sisterhood is designed to produce.
None of this is an argument that the later books are bad. They are uneven, occasionally over-explanatory, and they inherit a great deal of the original's furniture without always earning it. But they are the only place in the whole sequence where the plan is seen from the inside across generations, and the ache in them, the sense of people doing their duty inside a machine they cannot see the edges of, is the true continuation of the first novel's argument. The first novel asks whether the prophecy is true. The later books ask what it costs to be its instrument.