Not so likely. The other reasoning behind wood in most cases was the more cushioning quality of it. The shield also has mass, mass reduces impact. (Ever notice how heavier firearms recoil less than lighter ones that fire the same cartridge? Same principle.)

Now think about it this way... why is catching a weapon on a broad, absorbative and heavy sheet inherently more dangerous than catching it with a lighter, inflexible weapon instead? It isn't. The parry would catch the same energy from a blow, and the wrist is weaker. Such a blow would only break a shielder's arm if they braced themselves in such a way their arm would break; this is unnecessary to brace that hard, because once the weapon is stuck the shield's job is done.

Also, not very many people were carrying splitting mauls on the battlefield, most weapons, including maces, were much lighter. A light weapon driven fast can break metal, against wood it's a little less efficient because of the softer nature. With metal, the initial moment of impact decides everything, with wood the carry-through is what decides.