I think I might have trouble with that approach for "harder/denser non-fiction" because for such reading I might well want to stop and think from time to time -- maybe there's an argument I'm not convinced by and want to reason through step by step, maybe there's a mathematical calculation with a non-obvious step, maybe there's a clever observation that I want to think about the implications of, maybe I've just realised that I lost focus and haven't really taken in the last page, etc. -- and I'd expect the extra friction of pausing and restarting the audiobook to get in the way a bit.

I assume you don't find that; is that because you very rarely feel the need to stop and think, or because it turns out that doing so isn't significantly impeded by having an audiobook playing?

(More generally I'd expect that sort of book to have easier and harder bits and I'd likely want to read them at different speeds. But I could believe that the advantages of doing this are enough to make up for it being harder to do that.)

Disclaimer: I basically never listen to audiobooks, so my guesses at what might or might not work well for me are (1) sheer guesswork and (2) not very interesting except in so far as other people have the same sort of experience I'm guessing I would.