The size of the rotor and pads for one. Too small and you will have less brakes. Two, are you running a stock flywheel? It's really hard to slow down that rotating mass. Three, the strength of the master cylinder and the caliper piston, again too small and they will work great with no load but fail when needed at the higher end of load. Four the type of material the pad is made from. Five, oil/dirt/grease will hamper your braking ability.