Anything that has to do with wave motion physics: Snakes, chains, whips, stringy stuff. Warband cannot really support (without sacrificing a good portion of your life programming it). ALTHOUGH if they made a flail swing the same way everytime in the 3 swing directions, it could be do-able....But still a major annoyance.
Think of it this way, the model of the sword basically stays the same throughout its' entire motion, a chain does not.