As someone who didn't read the book, the dune adaptation felt pretty hollow. A lot of visual splendor, but a very thin story with very empty characters. Extremely shallow interactions.
Considering that, I think the interactions were too articulate. Trainspotting was Dune with less drugs and internal conflict.
There was a lot left unsaid in the film to keep it concise and uncomplicated for a viewer like you who hasn't read the books. (Perhaps it was an overcorrection.) The director would occasionally reduce a long meandering sideplot to a quick distrustful glance from one of the characters. Someone who's read the book would pick up on it and feel like it did the book justice by acknowledging those events rather than cutting them out entirely, and the film doesn't burden viewers who haven't read the book with an impossibly hard to follow plot crammed into 2 hours.
And what might be attempted once might receive adaptations again, until someone gets it right.
For instance, in my earlier post I mentioned the live-action Ghost in the Shell, which in some parts Hollywood almost-mechanically adapted to be shot by shot identical to the original animated film, while completely missing the point with the racebent casting choices and much of the writing. I would count that as a poor adaptation, but not adapted so faithlessly that it has no resemblance to the original work.
Then again, Blade Runner is a loose adaptation of PKD's original story, yet does its central themes justice. So I guess "unflimable" is a relative term. Maybe it could ALSO just mean "no demand or desire to see adapted for film or television."
My overall point is that it's a golden age of IP and with the mainstreaming of nerd culture (and the endless demand for content from the streaming studios), all sorts of works that were once considered unfilmable are about to see the light of day.
This is the first time I've encountered this term! What an excellent word.
Put that in Foundation's movie reel and film it!