What holds back refinement of self-energizing bicycle brake designs?
Power assisted brakes that amplify the amount of force applied to a bicycle wheel hasn't taken off because the idea doesn't provide a large enough a benefit of increased stopping power and saved energy required to stop to counter the cost of R&amp;D.  

Cars have mechanical assistance because of the amount of force that is needed to bring that much mass to a stop is not possible with the amount of leverage and force a human can safely do alone.  To get the vehicle to stop in a safe distance, you need to exert a huge amount of force onto the wheels to get the car to stop.

Bikes with their human passengers require much less force to stop.  Stopping power is sufficiently amplified through mechanical leverage and friction materials - enough to easily lock up a front or rear wheel with a slightly tighter squeeze of your hand.

My impression of an enhanced brake system on a bicycle is that your range of power application would be narrow due to the limited range of motion in your brake lever. Your brake lever already moves so slightly from no-braking to full lock that a device that amplifies the amount of power applied throughout that range would send your brakes from 0 to 11 near instantly.  Of course you could tweak the device to be more sensitive - or - just use mechanical brakes and not worry about a more complicated system.