CONTAINS SPOILERS
I saw an interview with Frank Herbert around the time the 1984 movie was released. One of his statements stuck in my head. "Most authors wonder what [the people in Hollywood] are doing to their baby. I know because I was directly involved the whole time." Or something like that.
IMO, his direct involvement was a problem. The novel was longer than could be contained in a standard-length movie, yet even when things were cut, nothing was altered. For instance, in the book, Paul has a knife fight shortly after he and his mother are taken in by the Fremen. This knife fight triggers a sequence of events, including his becoming responsible for the children of his opponent's widow. In the movie, the knife fight happens, but many of the events, such as the presentation of the widow and her children to Paul and his decision of how to accept them, that followed from that were cut. Yet, in the movie, there are still these two young boys who are following Paul around.
The result I found was that if you had not read the book, you could not fully understand what was happening in the movie.
My poor brother actually took me to the 1984 movie as a birthday present. He had never read the book. At different points in the movie, he would lean over to me and say things like, "Where did those kids come from?"