The balancing factor of spirits is that they are NPCs, not PCs, and they are controlled by the GM. Just like any other non-player character, you have to negotiate with them to do what they want, the only difference is, spirits are obligated to obey, even if they don't want to. So if you force spirits to do things that they disagree with, or abuse them as bullet shields and such, your mage develops a reputation in the spirit world, and that will come back to bite him in the ass.
Spirits can get ridiculously powerful, but the drain is brutal to summon one that outstrips a well built PC, and they are inherently temporary and generally won't like it if you make them fight while you run away. I personally don't see them as unbalancing since the GM has a multitude of ways of punishing a player that tries to abuse spirits, and in my experience, most players are reluctant to call up an NPC to do all the fun stuff for them, which is basically what summoning a spirit is.