They largely explain things that, as far as I'm concerned, didn't need explaining. As you probably know, they wrote a 2 book conclusion to the series. These 2 books largely tie the Frank Herbert Dune universe with the B. Herbert/K. Anderson Dune universe. And that seemed quite forced to me and, cynically, felt like an attempt to force you to read their Dune prequel trilogy (because otherwise, you'll be WTF is Erasmus?) I feel like Frank Herbert, being an immensely better writer, would have been able to conclude the series without forcing a prequel trilogy onto readers.
It wasn't until I read some of the more immediate books (universe time-wise) from the duo that I came to appreciate how effortless Frank Herbert's writing is. Gurney Halleck is loyal, he just is. And that loyalty is clearly based on respect for the Duke, being on the side of "good", and a hate for the Harkonnen. We don't need a justification for Halleck's devotion. It's pure and effortless character development. The duo doesn't have that seem ease of writing, so instead we get the back story, torture and rape (lazy themes) that tainted the character/original work.