You really cannot compare lesser summons to a level 9 spell. It's a whole different game when the high level stuff is researched.
Ghost Riders is a great spell and it does do good work. I doubt anyone is arguing that's it's weak.
However if you compare to other 9th level spells, you will find that the power level is not inappropriate.
We are talking about the same level of research that brings us effects such as;
Wish - Gems, Doom Horrors, you name it!
Utterdark - The whole worlds income reduced by 90%!
Flames from the Sky - Blows a chunk out of any army, anywhere.
Arcane Nexus - I can always use a few hundred astral gems every turn.
Yes Ghost Riders is strong and efficient, however we are talking about access to the most potent spells in creation.
The most capable death casters in the world can send an impressive undead force to strike at your lands? Sounds about right to me.
