What rolling b said.
Consider that the substitutes are tremendously hygroscopic, they will suck water out of dry air.
The only real positive thing I can say about Pyrodex, et al, is that they are considered a propellant, not an explosive, and as such, are easier to find. Most everything in Alaska has to be barged in, and it just depends on whether the barge company wants to haul explosives.
If you can get the real deal, go with it. Forget the phony substitutes.
John