I didn't quite mean it like that. When someone is designing a video game's characters and enemies, they must be careful to preserve a balance to prevent gross imbalances in the combat system.
As an author, you have a similar obligation, although instead of stat values, you're dealing with something more abstract. Sure, you could easily allow your magic users to wave their hands around to soar through the skies, revive the dead, or make the enemy army's heads implode--all with virtually no effort. Of course, this creates the problem we're trying to avoid.
Instead, flying through the air might require rest if done for more than an hour, reviving the dead might require the sacrifice of something that is already living--quite likely the caster's own life, or your character's magic might only be strong enough to make a few soldiers implode.
You just need to be clever about how you limit things. That's all.
Edit: As a sort of practice story, my protagonist belongs to a group of summoners who lives in village hidden away from the wrath of the world's self-righteous church. So she's able to pull fiends out of a hat at the very least. In addition, she has the unique ability to work magic without the world's normally-required spell incantations, and on top of that, she was born with innate combat skill and strength that is above average, despite a lack of training.
As you can see, this leaves an amazing potential for imbalance, but as the writer, I plan on making sure that's never a problem. In fact, I gave her all that badassery on purpose--since it's a practice story, things like balance, a viable explanation for having above-average power, and making the character a person rather than a set of abilities is all something I wanted practice with.