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, vulnerability to buffs 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. Nature and Death will also slow them down so much that they will only reach the end of the battle. And these buffs might not even drop in the Magic Guild - so what do you do then? P.S. Titans on the Tower generally destroy everyone, but there's a catch. And Hydras definitely won't reach them in time, unlike the Dragons.
And Ifrits are good guys, I agree. But Nightmares are not inferior to them in battle and combine much better with Super Archers and Medusas.