[st.patrik]

What about making the enslave mind have a very high fatigue cost, or something like that?

Just to explain why I thought it was overpowered - I played a game vs. AI as Arco, and was attacked on several occasions by small Groups involving 5 Hama Dryads. I had a much larger force, but ended all such engagements losing most of my guys without killing a single one of the enemy. I agree that it would be hard to use this strategically, with the need to build labs etc., so what I am more concerned with is the defensive effect of having, say 10, dryads guarding each of your fortresses. stick in a couple of heavies to jam the doorway and your fortress is never going to fall - especially if you throw in a staff of storms to make sure flyers can't get in.
Also a handful of hama dryads would be worth sacrificing when used offensively, if they are basically able to take out your enemy's army (and probably win you some new troops (worth more than the summoning cost of the dryads themselves) in the process.

What about instead of giving them a ranged attack you just made them a lot tougher in combat - higher defense score, perhaps a life-drain melee attack, or something like that?

Other than this detail I like a lot of what you've done Zen. Have the devs tried this stuff out, does anyone know?