The pressures generated by black powder can be every bit as high as those generated by some smokeless loads; some smokeless loads generate less pressure than some black powder loads. There are smokeless loads that routinely generate MUCH higher pressures than any black powder load.
Some 19th century muzzleloading target rifles using black powder generated enough pressure that conventional percussion caps could not be used. Sealed primer ignition systems were necessary.
There have been modern muzzleloading guns designed for use with certain smokeless powders. I know that a modern muzzleloading 24 ga. gun that we made was fired with components from broken down 20 ga. smokeless shells, without mishap. Is there any reason that a muzzleloading shotgun with a modern steel barrel and modern steel breechplug would be less strong than a single shot Cooey with a mild steel barrel and a cast semisteel receiver?
Black powder used in a muzzleloading gun is rather predictable, and perhaps this is why it is THE best choice.
There are many smokeless powders, many of which could seriously damage a muzzleloader - or a breechloader, if used incorrectly, as far as that goes. Load a .30-06 with Bullseye?