The only limit to black powder is reload speed. And, heck, you can carry spare cylinders if you're all THAT worried about it as with Pale Rider.
The main reason NOT to carry BP for me is I don't live in the country where I can shoot them empty and reload with regularity. Also, there's far better choices in powerful modern handguns that hide better. Texas has no open carry. I do think I'd like to add a 5" Remington or maybe a short barreled Navy to my little BP collection eventually. There's a 5" remmy on sale at Cabelas right now that's tempting. This would give me something big that's more compact than my ROA and more powerful than my .31. But, it's not a real rush since I carry a .357 magnum and a .38 and plan to keep on keepin' on. I do have autos, but I sorta like my revolvers. And, firepower isn't an issue in self defense, as GOC points out. I just LIKE and always have, .38 and .357 revolvers.
I have the money at the moment for one gun. I've got all the cartridge guns I want and need for the moment. I thought it over, don't really need or want a shotgun. I'm a big time bird hunter, mostly waterfowl, both geese and ducks, and a dove hunter. I have two preferred shotguns for dove, one I can use that I've sort of retired. I have my favorite Mossberg pump for ducks with the Winchester 1400 (dove gun) capable. Then, I bought a 10 gauge H&R two years ago for geese and LOVE that thing. Talk about reach out there and knock 'em down! I've had to re-adjust my range limit thinkin' with that gun over 3" BB steel. I'm shooting 3.5" T shot out of it, friggin' cannon.
I absolutely don't need another big game/deer hog rifle. I don't have a lease anymore and my hunting is restricted to my place and stand. I can't see more'n 150 yards. Heck, most times I hunt with my contender pistol to put some challenge into it and am getting into bows again.
I really don't need another carry pistol and don't have any other handgun wants. BUT, I had thought about getting a micro .380 like the Ruger LCP. My buddy brought his new LCP down and I compared my old Grendel P12 to it and by danged the Grendel is the same size! It's a few ounces heavier and 1" wide vs about 0.8 for the LCP, but it has a double stack 11 round magazine, so the width is pretty worth it IMHO. That took the sting out of wanting an LCP, though.
So, then I got to thinkin' about what I'd want in BP since my BP collection consists of 3 guns. I coulda ordered the Remmy Cabelas has on sale, but I have, over the years, thought a lot about playing with a NAA mini Companion and that I might be able to get more out of it than my .22 and it might actually become a carry option. If I can do that, get it as accurate and more powerful (aka significantly better penetration) than my .22 NAA, I will buy a couple of spare cylinders for it. I'd thought of getting a .22 mag mini master, but the things have 4" barrels on 'em. They'd still fold up pocket size I reckon, but I've been unable to find one locally. I may still get one someday since it seems I'm building a small NAA collection now.
But, a shorter barrel fits the carry thing better. The .22 mag, though, really needs 4" of barrel to be much better than a .22LR for defense. All it offers in the short barrels is a little over 100 fps more and a LOT more muzzle blast. Thing is, if I can get that Companion to perform on par with the short barrel mag, it'll be short and handy and more fun to shoot and play with and cheaper to feed and, well, it'll just be neat.
I won't carry it daily as a back up as I do with my .22, requires unloading and reloading regularly. But, if I get into a situation where even my .380 is too much to conceal, rarely happens, but does, the little Companion could be an answer. If it don't work out, hell, it'll still be fun to play with and I'll likely keep it for that, anyway.
Anyway, all that was my thinkin' on the subject, twisted as you might think it.
Funny, with cartridge guns I tend to try to justify a purchase. I like Browning pumps, BPS, and thought about saving up for a 10 gauge BPS, but money is a little tight and, well, I don't NEED it. It would displace other shotguns for waterfowling that I like. With black powder guns, though, I need no other justification to own them, really, other than pure fun. That's what BP is all about for me. So, i really can't go wrong getting a little NAA companion to play with way I see it. It may turn out not to be such a practical tool in the end, but still, it'll be fun.
That NAA mini master might offer better accuracy through being a lot easier to shoot. I may get one one of these days. I do have a little stainless Rossi 511 that's a tack driver, squirrel hunting accurate, that I carry when I'm fishing sometimes. The mini master would have to offer a lot to displace that gun as a kit gun. That gun has a 4" barrel and is J frame size, rides IWB very nice.
So, well, I'm happy with my decision. It's been two years since I bought anything new to play with and, hell, I have the urge for something new, don't really care about need.
