From what I recall, the Auto Brake system, set at anything less than 'MAX', gives a certain rate of deceleration NOT a fixed amount of braking pressure (got them written down somewhere). Thus when using Auto Brake PLUS Thrust Reverse the Auto Brake system bleeds of the brake pressure to maintain that given rate of deceleration.
Perhaps this is why it is still controllable with N-1 reverse thrust. I dont know if the auto brake system is sensative to assymetry or not (ie deceleration per wheel bogie or just total deceleration).
MAX Auto brake plus Reverse Thrust might be a different story!