Do you feel the same way about the Lord of the Rings films? The most common complaints about those are where it has strayed from the source material.

Dune isn't fully pure, either. It goes against the most frequent theme of the first book simply by 'shooting' scenes outside during the day. Caladan is much more fleshed out. It introduces new Bene Tleilax lore in the first film which is notable because Herbert himself didn't introduce them until there was a second book to write, omniscience to restrict and characters to resurrect. Looking at the trailers for the next film, Feyd Rautha is bald Elvis instead of a handsome Paul-like character and I'd be very surprised if all of the Baron's proclivities are retained.