In the book, it seems pretty apparent that ‘Spice’ is a metaphor for ‘Oil’. In 1965 it was apparent to the brighter folks that the Arabs had a vital resource greatly under their control; the political, economic, and martial implications of that were part of what Herbert (who admitted to being an admirer of Joe McCarthy) was exploring in the book.
However, until we get to other planets, we won’t really know, will we? It seems the least plausible part of the entire book to me.
There’s also the back story of a war between humans and AI that humans barely won, something that movies have a hard time covering.
Of course the worst abuse of human modification came from the Reverend Mothers with their master plan of eventually taking over the known universe through their selective breeding program. Here's a story fans of Adolph and Margaret Sanger can root for - until those plans collapse upon themselves at the end.
Interesting choice of Walken as Shaddam IV. He's good but can't escape the "Duke Syndrome" (no matter what role he took John Wayne was always John Wayne). I'd have gone with Mark Hamill who, besides being old now, morphs so well he's sometimes hard to spot.