Isn't a case full of black a "Cowboy Load"?

I know they mean them pip squeak SASS rounds. I still don't see how a black powder round could generate the pressure needed to hurt anything. I mean what's the difference in me shoving a 240 grain Kaido on top of 30 grains of powder and shoving a 250 LRNFP on top of 30 ish grains in a cartridge?