Except Brian Herbert was writing books before he started writing Dune with Anderson, and has written other books since then.Well, if you're sitting on your dad's very valuable IP you may as well cash in, and if you've got no writing talent, you can always licence the writing part out. That brings in Kevin J. Anderson: I've read one his novels, and it convinced me that I never need read another.
That said, reading Sisterhood of Dune now, and having read most of Saga of Seven Suns (also by Anderson), it's got Anderson's fingerprints all over it. Worldbuilding is fairly good, but the writing is functional more than anything else, the characters are fairly bland, and there's some bizzare choices of language, such as when the characters refer to "the evil [thinking] machines." This is primary school-level language. Secondary at best.
I'll be honest, you're probably not missing out on much by skipping it, and assuming the level of quality is constant, probably the rest of Expanded Dune as well.
Is it actually confirmed how much of each author writes the books?B. Herbert could definitely have done far better than Anderson, indeed. I have no doubt F. Herbert had plans and notes for beyond the seventh novel even if he clearly peaked with God Emperor and kinda descended into weird pulpy psychosexual shit with Heretics and Chapterhouse, but what B. Herbert and Anderson did with it was ridiculous.
I can't comment on Brian Herbert at all as a writer, but Anderson...well, as I said, Sisterhood has multiple similarities with Saga (even down to plot points, arguably), so I could imagine that it's Brian doing outlines while Anderson does the actual writing, but if so, I'm left to ask why.
The weird thing is, I'd say Anderson's got the opposite problem. At least going by Saga, he's very good at worldbuilding, but he can't really create interesting characters to inhabit those worlds, and the writing is all function, no form. It makes me want to see Saga adapted into a TV series or something - streamline the plot, get a better writer on hand, bingo.