Woodruff keys in a motoring application usually fail through design error or wear. They are used on tapers to locate the male and female parts and not to drive. On a parallel shaft, they drive but if there is significant wear between male and female, torque reversal will fatigue and break the keys. Four fairly small Woodruffs copes with the torque of a big vee twin on a Morgan even on a hill climb where revs vary wildly and wheels sometimes grip and sometimes spin. If you are not using splines or squared shafts, a round taper pulled up tight would be enough for a bicycle crank but it would still need a key to locate the cranks at 180 degrees to each other.
And how can they be 'very early cotterless cranks'? Shouldn't that be 'fairly early'? Pre WW1 Rudge would be very early.
Stuart Tallack raising an Ulster hand in West Sussex