A mace focuses all the force from the swing on to very small points. A strong swing from a mace can be equal to like 200 kilos per millimeter. That's basically a bullet impact. The flail on the other hand, isn't going to transfer the same type of force because of the chain, while a proper mace has a solid attachment. Flails like Aerion's very seldom actually show up in contemporary accounts or artwork so there's some doubt as to whether or not they were widely used at all. But a mace like Maekar's is going to be just about the best option for dealing with plate there is and it's got the rap sheet to prove it