Well, when a motor shorts out(& there are a few ways it can happen, even physically locking one up will have similar results), its amp draw will spike at CONSIDERABLY higher levels than 120-150 amps(at least the serious mod motors will, the smaller 540-sized mild wind spec motors will draw closer to 120-150 amps max. under such conditions), so if an ESC can't handle such spikes it can be damaged more easily.. Not to mention, every motor is a little different, even with 2 motors of the same wind & type, manufacturing tolerances alone will produce 2 motors that draw a little differently from one another, so some will be more efficient than standard, & some less so. Just remember that it is ALWAYS good to have room for emergencies when looking at ESC specs....