generally speaking, you can use a roof membrane on a wall but not the other way round. you're right that the the fundamental tech is the same, but the performance criteria for roofs is much higher with regard to permeability and water holdout. it's actually pretty common to use roof membrane in walls, particularly on smaller projects, or where stick building to use smaller roll widths.

wall membrane on on a roof however is a definite no i'm afraid, building control would have kittens, and technically it's not wise either.