I would disagree. The sequels, especially 5-6 open up the wider universe and explore a variety of human cultures and social interaction. It's a fantasy-scify with deep and complex characters and complex interactions. Normally you'd start to see a scifi universe break apart after a few books, but Dune's could stand, it's 'realistic' in a sense.

If you mean the sequels, prequels, interquels made by Brian Herbert and Kevin J Anderson, then those are of a lower quality. They had the good fortune to inherit an IP and just ran with it. They may not be so bad as far as the genre goes (except for Sandworms), but they create a somewhat wider context. It's not for everyone, while I feel the originals of Frank Herbert may be, at least to people with a taste for sci-fi.