From Wikipedia:
Don't know if that explains any difference between 3D and 4K, but it would seem to explain why the overall film appears a bit "softer" than some expect. It sounds like the final film is basically sourced from a 35mm print, rather than from original footage shot by the cameras.
I watched this in 3D a few days ago. 3D is not essential, but it adds some nice depth. I enjoyed the film (my first exposure to anything "Dune"), though I'm still not sure I understand all the politics and mystical mumbo-jumbo. Maybe a little bit more explanation would have been helpful. I also...
And if the concern is the use of actual IMAX cameras during filming, that's on the creators/director, not the theater showing the film they're provided.