Anderson wrote crappy Star Wars books, too. I'm really not sure what the breakdown is of Brian Herbert and Anderson actually doing the writing.
I gave the two of them two or three trilogies, so more than a fair shake. Painfully dull is the first adjective that comes to mind. They really took some of the most interesting places and concepts in sci-fi and made them flat.
Hunters and Sandworms were the least bad books by the pair, and there was certainly some hint of Heretics and Chapterhouse in them, but they were still not well written and the ideas lacked the substance of Frank Herbert's works.