The diaphragm PP's spring design typically starts out gaining clamp pressure as the disc wears, then at about the mid point of disc wear the pressure curve levels out and begins to gradually lose clamp pressure for the rest of the disc's life. Basically you end up with about the same clamp pressure at the end of the disc's life as you had when it was new.
The B&B and Long designs use conventional coil springs, which only lose clamp pressure as the disc wears. That means you must start out with a wide margin of extra spring pressure when the disc is new, just to have enough clamp pressure to hold at the end of the disc's life when it is thin.
Hypothetical example, lets say a given combo needs at least 2800lbs of clamp pressure to hold a given engine's torque.
...With a diaphragm setup you might start out at 2800lbs with a new disc, gradually increasing to about 3150lbs at the mid point of wear, then gradually decreasing to about 2800lbs by the end of the disc's life.
...With a Long or B&B, you would have to start out at about 3500lbs with a new disc in order to still have the required 2800lbs when the disc is used up.
Diaphragm starts out at it's minimum pedal effort, while the B&B starts out with it's maximum pedal effort. Obviously the B&B clutch pedal is going to feel quite a bit stiffer when new compared to a new diaphragm setup of similar capacity.
Grant