You’re correct that tumbling or vibrating loaded ammo that is intended to be fired is a very bad idea. If done for a long enough period it can break down the powder into smaller granules. 4831 can turn into Bullseye. There was a reported accident several years ago where a Ruger 44 Magnum revolver burst, severely injuring the shooter. It was found that he had carried the revolver, loaded, in the glove box of his pickup for a couple of years and most of his driving had been on gravel roads. All of that vibrating had reduced the powder to not much more than dust.
Tumbling lead bullets or soft points will round off all the edges. Vibrating them with any kind of course media, such as ground walnut shells, will leave them pock marked like an automobile that had been caught in a hailstorm.
My above description involved only empty cases. I probably should have specified that.