I'm not even considering collaboration/conflict between ESP and the retrofit. A car whose chassis is designed to have force applied on the front wheels only (hence with rear wheels freewheeling) may not react properly at all to rear wheels having power. From suspension to weight distribution to force transmission through the chassis rigidity.
For example (oversimplification), FWD cars have their suspension balanced to be stiffer on the rear because it gives the front more grip (and the reverse is true for RWD cars). This stiffness would not usually result in a loss of grip under load (e.g in a corner), but adding a force to the wheel may overcome the remaining grip and throw the car into a tailspin.
Applying a rotating force on the wheels creates a pinching effect, momentarily changing the parallelism. This effect is sufficiently noticeable that some cars are designed with wheels not parallel at rest, so that when you drive on the highway (and thus apply some power to the wheels) the wheels are parallel.
There are countless scenarios where things can go wrong because you're simply doing something that the car was not designed to handle.
(For the curious, the Forza Motorsports series has a nice sandbox mode allowing to experiment with various settings by changing them on the fly, and the inline help explains succinctly the impact of each knob on your car's behavior)
But these electric engines could be programmed to only run when the car is moving in a straight line. If the wheel speed between the two rear wheels starts to differ then the motors turn off.
