[Edi]

If Ghost Riders behaves like that consistently with everyone, then it belongs in the list. Can anyone else confirm this behavior?

One question I would ask is how big a game is it that you're playing? Because that sort of behavior would start happening if the game had reached the unit limit and/or the commander limit. There are only so many units that can be on a map all at once and only so many commanders. Once those limits are reached, new units or commanders can no longer be recruited and can no longer appear from spells such as Earth Attack, summons or Ghost Riders.