I was just thinking about Dominate, versus the other Level 3 spells (basically all summons.) Domination is basically, at the minimum, 3 times as powerful.
When you summon, you create something like a Djinn. It lasts for 1 turn unless you have a summoner trait. With the Djinn/Earth Elemental/whatever, you assault the enemy stack, most likely killing something.
The gross effect: enemy -1 unit (strongest defender); player no change
The net effect: power balance shifts 1 towards player
Now, when you use Dominate, you permanently take control of the strongest unit in the enemy stack. Many times, you can attack with this unit the turn you get it. If you do this, most likely you will kill off or seriously damage something.
The gross effect: enemy -2 (domination, killed); player +1 unit
The net effect: power balance shifts 3 towards player
This makes Domination 3x as powerful as a level 3 spell from another sphere. On top of that, it ignores the fact that you can continue to use the Dominated unit in subsequent turns without having to recast Dominate. (Which makes Dominate have a snowballing, quadratic effect, while the others remain linear.)
It seems to be overpowered relative to the other spells.
What if Dominate lasted only 1 turn, and reverted control in the same manner as Enrage? By my math, you can take control of an enemy unit, then kill something, damaging the unit in the process. Then they get the damaged unit back.
The gross effect: enemy -1.5 unit (weakened, killed); player no change
The net effect: power balance shifts 1.5 towards the player
This further reduces Dominate's effect to something linear because the Archmage has to cast it every turn. It's still more powerful than the other spheres, but at least it's more comparable this way.
I think it would be more balanced this way (assuming failed Dominations no longer caused the caster to lose the promotion.)
Also, thematically, Domination is supposed to take concentration by the domineer. It doesn't really make sense for the caster to be able to dominate as many as he or she pleases without some at least of them breaking away, unless the caster is Perpentach himself.