My male BP is on cypress mulch and has heavy rock hides but he also is able to lift his hides while underneath them and move to a more desirable location. I've seen him do it many times since he has a pretty large UTH on the warm end of his enclosure which has slightly varying temperatures across it (I always make sure to measure the temp of the warmest part).

I do think flat substrate has some advantages like being easy to clean and good for monitoring the snake's waste or seeing parasites if there are any, but I don't know if this particular advantage is exclusive to flat substrate.