Not quite true. Jackson and co.'s changes seemed to multiple upon each other as that trilogy went on. If you directly compare Fellowship and Dune Part I, I think the former compares favourably with the latter in terms of faithfulness.
I need to see it again, but as much as I really enjoyed the movie and thought it was a beautiful bit of filmmaking, certain cuts bugged me. There was a part of me that found the narrative too streamlined and potentially lacking in depth. For instance, I would have loved to have a more fulsome plotting session with the Baron at the outset as in the book. I think it would have helped some viewers get into the world more and it would have been a wonderful moment to really establish the Baron as a wonderful classical scenery chewing villain.
Second, I found the cuts to Dr. Yueh's story hurt the narrative a little and I very much would have liked to see the dinner scene. In particular, I worry that the cuts to the traitor subplot will undermine certain character decisions later. It will arguably result in bigger changes in Part II, just as small changes in Fellowship begot larger changes in Two Towers and Return of the King.
We can only judge the faithfulness of the adaptation when we see the story in full.