I think the issue is this mystique surrounding the idea of Dune because of how long it's been around, and many reading it when they were either young or just getting into reading. All in all, it's not as deep a story as people make it out to be. The characters are all mediocre at best and the plot is fine. It's a good sci-fi story.
However, because of the perceived weight behind the book series, some expected one of the greatest character studies and stories in history... which it just does not have. I only read half of Dune and thought it was fine, but nothing about it carried the same weight I felt when first reading something like The Lord of the Rings. I may finish it one day, if something drives me to pick it back up.
At the end of the day, the world and visuals Denis Villeneuve gave us were, in itself, greater works of art than the book, imo.