As a rule, Hydras are weaker than Black Dragons. You can choose them against Barbarians, or when you lack resources for Black Dragons, or to surprise your opponent, but... firstly, they are slow. Secondly, their susceptibility to enhancements is good, but against, say, a Haven Mage, Hydras are a death sentence for yourself. They will be hypnotized, slowed down, teleported, and subjected to the Song of Peace, etc... P.S. Titans on the Tower generally defeat everyone, but there is some confusion. And Hydras definitely won't reach them in time, unlike the Dragons.