I think muzzle loaders would be pretty crappy in the game. At first you might get a super awesome feeling of being awesome, then you would discover that it svcks too reload, get appropriate amount of gun powder, poor it in barrel, get a lead ball and put it in barrel, take rod out push ball in and put rod back in, add the cap, and fire your one shot. Pretty ineffective if you ask me.
It wouldn't necessarily be a gun you could use like any other, it would be unique. It could be used for a more stealthy approach to an enemy, since you do only have one shot, and that one shot, for all you know, could be the one that puts down the enemy for good, and if it doesn't you've got the rifle for a swinging club.
Plus, even with all the time it took to load and prime one of these, they were a powerful rifle. The Pros to it would be, it's lighter than say a Anti-Material Rifle, can knock down something as powerful as a Deathclaw, like an Anti-Material Rifle, and of course is multipurpose where a Anti-Material Rifle isn't. The Cons would be it's loading process.
Crossbows on the other have that multipurpuseness. And i just wan to stick people to the wall like the highway rifle
I don't think crossbows shouldn't be in Fallout, I even thought they should be in, when I was playing FO2 brand-new, out of the box. But something that has better range and power, but isn't a more modern rifle, would be something I think could add something to Fallout. And like I said, it would be a tribal thing, not really something a modern wastelander, would use.