Exactly the same symptoms as one type of "Loss of Tail Rotor Effectiveness" (LTE). If you let too much yaw rate build - with the tourqe - you will not be able to stop it, even with full application of opposite pedal. The only recovery is to get some airspeed. Prevention is better than cure! turn against the tourque.
I would imagine that's the same with Fenestron systems.
Does the Fenestron suffer from a problem associated with main rotor vortex interference, the same as conventional tailrotors?