I've been shooting BP in competition for over 20 years. I used to shoot 1000 yard matches with a Sharps and am a Cowboy Action Shooter, shooting 2 to 3 times a month. I shoot somewhere around 2000 rounds a month in matches and practice. Granted I shoot BP cartridge mostly, not M/Lers. But cleanup is very simple (I use water and a touch of simple green). I actually think BP loading is easier than smokeless. Think about it, had to be so the cowboys and buffalo hunters could load their own without a lot of tools. I have also never had a problem with misfires/nonfires when I use my front stuffer. Sure its not a flintlock and does use 209 primers, but if you follow a few simple rules, you won't have much of a problem with ignition in wet weather. I use a balloon over the muzzle to keep the water out...remove prior to shooting of course. The biggest problem I have with moisture is with the trigger mechanism, get a film of rust on it and gets sticky when it gets wet. Thats why I'm getting a new M/L.
The big advantage to M/L hunting here in Colorado is that I get to hunt during the rut. I'm not a bow hunter, never was and never will be. The biggest problem I have see is the attitude of the bowhunters, they don't want the M/L hunters to intrude on THEIR season. Sharing with them is ok with me, hell they could even bowhunt during rifle season too. Makes me no difference.