There has to be a distinction made between shooting smokeless in the two types of firearms - muzzle-loader and cartridge - that were originally designed for black powder. If you understand how smokeless powder burns and generates pressure, there are safe ways to load cartridges with smokeless powder and lead projectiles, and shoot them in black powder
cartridge guns. The critical issue is having a combination of powder burn-rate, air-space and projectile resistance (mass and engraving/frictional pressure) that produce peak pressures that are within the firearm's safe breech and barrel pressure limits. This can be pre-calculated with some accuracy if you have the know-how. I have done lots of experiments using 45-70 and 50-70 with cast boolets and various smokeless powders, and in the end, the ballistics are actually often better, i.e. more consistent muzzle velocity and subsequent trajectory, with black powder or Pyrodex! The real challenge becomes the lubrication (bullet lube, grease cookie, etc.)
There's
much greater risk with muzzle-loaders, even if you weigh out a "safe" amount of smokeless powder. This is because in the majority of cases, smokeless powder needs an air-space behind the projectile, and IMO this is too difficult to precisely control with a muzzle-loader. This air-space is a complete no-no with black powder, of course, but a must with the kind of smokeless loads we're talking about (peak pressures under, say 30 Kpsi). The amount of space will greatly influence the peak pressure.
So for the brand X .50-cal smoke-pole, under a 185-grain ball, you'd need N grains of "WYZ-BANG" smokeless powder, for example, and you'd need to seat the ball, say, exactly .321" above the smokeless powder. You would have to somehow set up your ramrod to do this accurately every single time. To me the whole thing just sounds like an accident begging to happen. Too much air-space, and the smokeless powder will not burn efficiently: you haven't blown anything up but bullet is stuck somewhere in the barrel. Too little airspace.... well, hopefully you have set your affairs in order. Is there an address to send a card or flowers?