In my experience, Pyrodex is much worse in this regard than blackpowder. Either can result in rust if the gun is not cleaned, but Pyrodex does it much faster. Stainless will resist longer than carbon steel, but not forever. Blackpowder seems to promote rust by attracting moisture, while Pyrodex seems to add an active component, which reminds me of an acid, to the equation in addition to attracting moisture to the metal.
I have seen dirty guns left with BP fouling over a period of months with no rust on them, and I have seen dirty guns left with Pyrodex fouling that were ruined in a matter of a couple weeks or less.
Neither is a problem if you clean in a timely and proper fashion.
