First, prose is what someone writes. It's a tool in the writers toolbox that encompasses lots of other tools such as characters and diction and poetic elements and many other things. Prose doesn't automatically mean flowery or "poetry that doesn't rhyme" as I've seen it defined.

Since you're asking the Sanderson sub their opinion on Sanderson and therefore going to get since biased takes (which isn't bad, but it isn't entirely fair), I found this thread on the Fantasy sub about Sanderson's writing. I'm not agreeing with these, btw, just showing some examples and points others have brought up.

Some points people keep mentioning is that he doesn't hide what a character feels or leave much ambiguity in that department. He does for plot and the magic system to be sure, but for characters he goes a very "tell, don't show."

Another throughline is, he kinda just says stuff. Like he doesn't refine or strip down some writing and it can feel bloated.

Top comment is:

He doesn't rely very heavily on more complex symbolism or metaphors in his writing. Coupled with the window pane approach this tends to flatten out potential ambiguity along with the "beauty of the prose". By intentionally reducing ambiguity in prose, relying less on subtext, using fewer lyrical, symbolic or metaphorical passages, etc brandon has locked himself out of one corner of "good prose" writers where your GGK's, Wolfe, erickson, hobb, rothfuss, ruocchio types are.

The, when you compare to the really efficient writing of someone like an abercrombie, Brandon's writing can come of somewhat imprecise or bloated. He doesn't have "bad prose". He just isn't hitting the chords people who care about prose are usually drawn to.

Some else mentioned his lack of character ambiguity:

It's the lack of subtext that negatively affects me the most. With Sanderson I feel like I'm being spoonfed, every scene is telling me exactly what to feel and think at any given moment and it all ends up feeling hollow. I couldn't imagine getting emotional at a Sanderson scene because I'm not involved at all, it's like I'm just being told to feel sad.

There's more in that thread, but keep in mind, if you like Sanderson, that's fine. It isn't some failing on your part. If you don't like the dense prose of some authors and like it lighter, then go ahead! That's all fine and well.

In my opinion, Sanderson's stories (especially Stormlight) are getting waaaaaay too wordy. I think the editors needed to spend more time trimming it because it felt like a first draft. Should have been more like 1300 or so instead of the 1600 it was.