Beyond the usual horribly unnecessarily distracting baggage that hydras bring with them ("My other head flaked so I can't answer why they said that", public intra-head arguments, "I read one of the heads as town, so I'm not lynching them", etc..), they disrupt the very idea of what mafia is about. Mafia is a psychological game where you have to develop the ability to instinctual read people for truthfulness and combine it with analytic reasoning. Having a second person to spitball ideas against is
CHEATING
plain and simple. Plus those that hydra will never get better at mafia because they will never develop skills on their own.