Now, it’s been a long time since I read the book, and I’ve never delved anywhere beyond the original novel, so treat this accordingly, but I think completely writing off the “science” of Dune is doing it a minor disservice. However, I’ll grant you that it isn’t “hard” sci-fi as that genre might typically be defined, either. Rather, the sciences that Herbert applied to his space opera were the social sciences, e.g. geography, sociology, history, and political science. I recall frequent digressions in the text talking about the ecology of Arrakis and how that has influenced interstellar politics, the most prominent example of this of course being the spice melange. Heck, the Liet-Kynes character was explicitly an Imperial Planetologist stationed there to learn about the Arrakis environment.

In fact, while checking my memory against the wiki article to make sure I wasn’t forgetting something, I saw this quote, which succinctly sums up my argument here: “the attention to ecology is hard, the anthropology and the psychic abilities are soft”.

All of which is to say, I agree with your main point: different stories require different approaches to scientific faithfulness. I liked all of the hard science of The Martian because, despite the fictional premise of an astronaut being stranded on Mars, I would largely expect an astronaut in that situation to think in terms of physics and biology and so on. I do not feel similarly about Han Solo. Right or wrong, I feel like authors with rigidly-held ideas about the value of hard vs soft science fiction tend to be insufferable pedants, and they write accordingly.