Is operator error the only possible cause of overspeed in the unfeathering process or can mechanical misfunction do it?
Ian, an overspeed can be either operator error, or mechanical error. Generally mechanical error.
The most common scenario for an overspeed isn't during an unfeathering process, but a governor failure in cruise flight. If you're flying the local club's Cessna 182, and notice that the RPM is increasing, check to see that the friction lock or vernier control is secure, and that the propeller control isn't creeping forward. If the propeller is approaching the redline and appears to be moving on it's own, you can try to control it with the propeller control, but you may have no success. In this case, your only choice is to retard power using the throttle...and begin to treat the propeller like a fixed pitch prop until you can get somewhere to land and have the engine looked-after by a qualified mechanic.
You seem very concerned about an overspeed when unfeathering. Can you share a little of what might be driving this concern?
Truthfully, overspeed is seldom an issue if procedures and limitations are followed, and the equipment is properly maintained. In most cases when an overspeed does occur, it's easily controlled. If it isn't, then in most cases a shutdown and slow-down can be performed, and the aircraft then reverts to basic engine-out procedures...simple stuff.