This article says shaft drives on regular bikes are less efficient--if it is less efficient, that might kill it.The controller is also build into the unit
The Protanium shaft motor is offered as ”one-unit” with the motor controller built in.
The system is equipped with a double planetary gear system which gives an exceptionally high torque. In addition, the device also includes two optical sensors: OSRAM. These sensors act as a speed sensor and a torque sensor. With these sensors we achieve the same pur¬pose as the mechanical clutches and therefore a dou¬ble security is built into the motor system, in order to ensure the rider’s safety. Safety is not only dependent on an electronic solution but also has the mechanical solutions as a back up. With optical sensors, we can dispense the motor assist exactly where the rider would rather have it, namely when start pedalling and going up hills. By dispensing the power in this thoughtful way we can conserve battery power and have an unusually high degree of power assistance.
http://electricbikereport.com/protanium ... ke-system/