Early in the game I usually get to watch my 3 spirits fall because the rook's swatted them out of the sky.

I don't think that Oak's minions are too strong, maybe the other generals need minions that are as useful as his, since he can cause people to move away from the fight, and push them away.  In other words he can move the battle forward, and maybe that's what all generals need to do.

However, I think he becomes pretty hardcore when he gets like, level 12+.  When he starts getting about 9 spirits with morale boosts, and his heal each time he kills something, he  becomes really tough to take.  Maybe he needs to be toned down a little bit in the end.

Also as long as you're moving, even as the rook, I don't think the spirits can touch you...just from what I've seen.