here's the rest of my "argument" about coaster brakes being prone to failure:
Rim brakes:
Brake pads can wear down quickly, and have to be replaced.
Rims should be checked for wear periodically as they can fail catastrophically if the rim sidewalls become too worn. Depending on the brake pads and rim, this can happen after a few thousand miles if heavily used in wet and muddy conditions.
The cables also can wear through repeated use over a long time, however they are more likely to get damaged through getting kinked or the open end becoming unraveled. If the inner cables are not replaced when they fray, they can suddenly break when brakes are applied strongly, causing brakes to be lost when they are most needed. Rim brakes also require that the rim be relatively straight; if the rim has a pronounced wobble, then either the brake pads rub against it when the brakes are released, or apply insufficient and uneven pressure to the rim when certain brakes e.g. dual pivot, are applied.
On heavily-laden touring bikes and tandems in mountainous regions, the heat build-up over a long descent can increase tire pressure so much that the tire blows off the rim. If this happens on the front wheel, a serious accident is almost inevitable. The problem is worse when descending cautiously at slow speeds because the brakes are "always on" and the cooling airflow over the rim is insufficient.
Disc Brake:
A disc brake puts more stress on a wheel's spokes than a rim brake, since the torque of braking is between the hub and the rim. The spokes therefore must be stronger, this leads to slightly heavier and more expensive wheels.
While a disc brake does not heat the rim, excessive heat build up can lead to disc failure. Bicycle discs are extremely lightweight, relative to the mass of bike and rider, compared to the large cast iron discs on an automobile, and therefore have little heat capacity. If brake friction exceeds convection and radiation losses, the temperature of the disc can very rapidly reach levels at which the metal begins to weaken and may warp or crack under braking stress. Therefore a disc is less suitable than a drum as a tandem drag brake, and heavy bikes (and riders) should choose the largest diameter discs available to increase heat capacity and cooling area.
Also, the hydraulic fluid may boil on steep, continuous downhills. This is due to heat build up in the disc and pads and can cause the brake to lose its ability to transmit force ("brake fade") through incompressible fluids, since some of it has become a gas, which is compressible.
(thanks, wikipedia)
what i'm re-iterating, is all brakes can overheat, and fail catastrophically.
i agree that having front and rear brakes is the safest route (although i don't, i just run a coaster,) but to condemn one over the other isn't justifiable. it still comes down to rider preference, and what you're used to.
and as far as stopping goes, a bicycle and a rider traveling at 35 mph, when confronted by a turning car right in front of you, no matter how good your equipment is, the car is going to be the "brake" that stops you. laws of motion dictate that even if your bike can stop instantly, you won't.
besides all of that, the main choice of MB's seems to be a department store cruiser, and in my opinion, those bikes aren't safe, with or without a motor.
i also want to add, that i'm not putting down any of the braking systems out there. discs are great, drums are cool, whatever. i'm just sticking up for the poor, often maligned, coaster brake. someone's gotta do it...