Im not sure what is meant by brushless being more efficient. It is better in most every way except torque density. Look up torque constants of same frame brushed versus brushless. You get more torque per amp out of brushed.

a magnetic field takes time to collapse and expand and its not the same in both directions. as the rotor moves faster it gets more important that the field collapses fast enough to avoid repelling the next permanent magnet. at higher rpms the field changes angle which creates more complication. dynamic timing is something a brushless motor can be setup to do. im not aware of a way to do that with fixed commutation in a brushed motor.

since continuous power is peak power limited by the motors ability to convect heat away... a rotor that is energized (brushed) absorbs heat rather than the case on a brushless. so heat is transferred to the air moving around in the air gap but the surface area is a little less than the case. So heat will build on the rotor shaft and be transferred to the bearings and then in turn transferred to the case. you usually have hot spots at the bearings. Peak power is greater in a brushed power but continuous is less because convection and conduction arent working as well as brushless.

Brushed motors have almost completely died in robots for primarily 1 reason. most people arent aware of the previous points. they died because of maintenance.

high end motion control is dominated by 2 motor types. variants of a brushless dc motor (synchronous) and variants of an AC motor (asynchronous). The ESC or drive that is providing power is synched to the rotor or not but the waveform is the same fundamentally. if you have an asynch motor eddy currents are created to move a non PM rotor. if you have a PM motor eddy currents are not created to move the rotor the attraction and repel forces come from an electro magnet and a permanent magnet.

is aluminum magnetic? look up some youtube videos and you will see some pretty interesting stuff that look like magic. an eddy current is created that puts the brakes on a magnet moving over aluminum. it doesnt repel or attract. I always wondered why people paid money for aluminum bolts for aluminum bolts in the motor can.