A Woodruff key is not man enough to transmit the torque required to turn the camshaft, it is there purely to make factory assembly easy. On the YB, the torque is transmitted by the bolt holding the sprocket onto the cam.

Years ago, I had a Suzuki GP125 which had a Woodruff key to locate the alternator rotor/points cam onto the end of the crankshaft to ensure the ignition timing was correct. It used to shear the key about every 500 miles as the tapered end of the crank was worn and not transmitting the drive. It only stopped shearing when I lapped the rotor onto the crankshaft with grinding paste. Never had a problem after that.