Is Ghost Riders too powerful ?

[Pocus]

I used extensively this spell in pbem. Yes it is powerful, and will allow you to do damages to enemy patrollers or small armies.

On the other hand, you wont do much against big concentration of troops. I'm not speaking of these useless units the AI build en masse, like 200 tribal archers, but of heavy mundane units and summon that players do possess by the time you reach level 9 in conjuration.

I dont find it disproportionate compared to the amount of research needed, and compared to what other magic schools can do. If you tone it down, then you will want to tone done mist warriors for Air, because it render a whole enemy army very tough, or suppress mass domination for astral, etc.
Each school, upon reaching the end game, has big and efficient spells. Its normal. You have paid a big cost and you deserve to have nice spells.