Author seems overly dismissive of the sequels. While they aren’t as great as the first, many of them are still very good books with interesting ideas. The stuff written by others (not Frank Herbert) on the other hand, does seem to be dreck based on my sampling.
As with the Matrix, the main problem with the sequels is that they deconstruct the basic geek power fantasies set up at the start.
No, the main problem with the sequels is that they add nothing to the lore. The first 2 books have a very detailed description of Dune universe and Arrakis and all the political factions. The rest of the saga brings nothing new or really interesting. They feel derivative. Nothing in the sequels have not been said in the first 2 books. So they felt already uninspired. It gets even worse after God Emperor of Dune, Herbert clearly had nothing more tosay after that.
Dune isn't a "geek power fantasy" Isekai style, it's a cautionary tale against blind faith in leaders and religion. It's made clear from the start that the protagonist, Paul, isn't a hero, as him and his mother manipulate the fremens to gain their trust.