People are referencing the S12's rock-and-lock as an inherent problem with shotgun detachable magazines. It doesn't need to be that way. Heck, you can buy S12 magwells, and the Russians even have some models that come with them. Same thing goes for loading on a closed bolt; don't confuse the S12's problems for "shotgun box magazines in general" problems.
It's also not as if speedloaders are an instant-fix to tube load times, either; they require some real practice to use well, especially under stress. Tubes can be reloaded quickly, but you're not going to be beating the reload time of a large magazine - again, especially in a super stressful situation. (If it was, you'd see integral-tube-magazine AR-15s, amirite?)
I think there's a better argument to be made that 12ga shotgun magazines tend to be rather impractical once you exceed five rounds; the sheer length of the magazine becomes absurd. Drums are better, but they're a political liability due to their "evil" looks, and they don't lend themselves to typical "tactical" reloads (ie, you'll probably need two hands for the operation).