Think about it. Thanks to the chain, it is very difficult or impossible to parry or even block with a shield.
i have seen some very odd smug blogs from historians who are doubting that the flail was ever actually used, despite a couple of paintings from the 15th century....because they just feel intuitively that it doesnt seem like an effective enough weapon.
why? what are they picturing in their head? Im guessing they just lack imagination.
picture that the shield blocks the staff part but, thanks to the chain, the ball curls around and hits the dude in the head. You cannot do that trick with a sword.