While I don't have a success rate percentage for you I gather it is quite high. I myself was NTR and passed the assessment a few years ago on the same day some TR candidates did not. I have met plenty of guys who have joined from Flybe/Eastern/Loganair and on the flip side I personally know two NTR candidates who admitted to me they became overwhelmed in the sim and did not get through. It is not a difficult assessment in my opinion but the usual things like showing an ability to learn and improve is the key I think.