I would say ideally, say for Dom 3, there might be a new property for which units have and need hearts or not.
Meanwhile, it's not the ultimate SC killer. I tried it as TC with abundant mages who can cast it, and it seemed to me it was being resisted by magic resistance, although occasionally it did some damage. It's an OK counter, but not an ultimate weapon. Ethereal crossbows and massed flame arrows worked better...
... at least without any missile counter-magic.
Frozen Heart is a really strong spell though... in Dom 2, I think it would be best if cold immunity AND magic resistance both offered protection.
PvK