A ton of people have already talked about this issue already so I will just say my piece:
Dragons (in most games such as this) are usually game breakers. In reality, if there were dragons in this time period they would destroy everything. The point I am trying to make is this: Dragons will always be difficult to balance. Either too strong or too weak. I think each dragon should be reworked for different uses. One dragon can be glass cannon, another tank, and another as the evasive mana eater. Allow them to be the masters of one form of power while severely lacking in other areas. It also makes sense to be allowed one dragon in each army. While all dragons can be held at home (for the achievement). The biggest issue is how exploitable the dragons can be, if they are allowed strength in one fighting style while weak in other styles, this will give the player a choice in the kind of dragon they want.
Glass Cannon: May go down quickly but can also destroy buildings/units/ etc as well.
Tank: May not destroy quickly but can act as a damage sponge for your other units.
Evader: May have low HP & low Damage, but can evade damage and poke the defending fortress.