Regardless of price, ease of use, or availability, the only thing that matters is effectiveness. I have experimented with both pelletized and loose granular substitutes, in cap locks and "unmentionables." Nothing beats real black powder in the right amount in the right gun. I have witnessed pellets following the projectile out of the barrel, burning and leaving a smoke trail. I have found pellets fifty feet in front of shooters, unburned or partially burned. I have seen shots that went "bloop" with the bullet hitting the ground fifty feet from the shooter.
My testing of pellets, from a bench, controlled conditions, chronographed for speed and targeted for groups has shown that in "modern guns" (the aforementioned unmentionables) and cap locks, loose powder beat them for speed, consistency, and grouping. Loose substitute powders also outperformed pelletized substitute powders.
Pellets are handy; fast loaded for second shot, and convenient. They are just less effective. They are expensive, fragile, and unpredictable. I want my second shot to be as fast and accurate as my first; although I can't ever remember shooting twice at the same big game animal with a muzzleloader. Second shots don't count. And anyone who says you don't have to clean up after substitutes is kidding you.
While I have shot, and do shoot all types of firearms, if I am muzzleloader hunting I am shooting real black powder in the correct load with the chosen projectile, usually out of a flintlock. And as a serious muzzleloading competitor, that goes double for a shooting match that I hope to win.