handbag,
With 2 spool turboshafts that have the power turbine in the back of the engine (ie. the opposite end from the PTO), it's usually very easy to damage the power turbine shaft with torsional overload. The reason is that the power turbine shaft in these engines tends to be very slender due to space limitations, and thus usually has very small stress margins in torsion. To avoid this issue, small turboshaft engines sometimes put the power turbine at the front of the engine (like the Rolls/Allison 250).
Gearboxes/sprag clutches/TR drives usually are designed more robustly than the engine. So these components are less likely to be damaged by transient overtorques. The only exception that comes to mind is if a helo has undergone a significant engine power upgrade with the existing gearbox, where the only thing preventing gearbox overtorque is the pilot and/or FADEC. This type of engine upgrade would be done to improve performance at hot/high conditions, but could also cause overtorque at SL conditions if not carefully controlled.
Hope that helps to answer your question.
riff_raff