I shot smokeless loads in a Ruger Old Army revolver a while back. It was a published load of Hercules Bullseye that was known to be safe in a .45 Colt with a round ball.
When I tried to shoot it, the cap popped and nothing, even though the ball disloged from the cap's explosion and jammed the revolver, the powder did not ignite. I used a ramrod to put the ball back in the chamber and tried again, still no ignition.
Finally, I got out some magnum caps and it ignited and blew the ball out, barely, it squibbed on me.
Then I figured, that in order for smokeless powder to ignite properly, it needs a powerful primer like is used in cartridges. So, I put in a small kicker charge of 4fg, about the amount that a .22LR case holds to do the job of a primer and success. The gun shot, it didn't blow up.
However, since I had to use black powder as a booster, I still had to clean the gun and so I saw little point in continuing the experiment and quit doing it.
Now, had I not been an experienced smokeless powder cartridge reloader, I could easily have assumed that the squib meant I didn't have enough powder in the gun and would have added more and more until it finally burned itself to non-squib pressure and then ka-boom!
It's not so much that muzzleloaders are too weak to take smokeless pressures, it's that a percussion cap on a nipple is a totally unsuitable way to ignite smokeless powder. If you just put a fuse into a stick of dynamite or C-4, or TNT and light it, guess what happens, it doesn't explode, it just sits there and burns like a pile of smokeless powder set on fire. To make it behave right, you need a blasting cap. The primer is sort of a blasting cap for smokeless powder, the primer sets up the initial chamber pressure that the smokeless powder needs to burn correctly.
A small rifle primer is powerful enough that a .22 Hornet case with only a primer, no powder, will propel a .22 cal air rifle pellet to nearly 400 fps out of a rifle.
Anyway, injudicious use of smokeless powder doesn't just blow up muzzleloaders, it blows up modern guns also.