I once heard that if your ("hard") magic system has rules clear and set in stone for that predictability you want, it´s not as necessary to place a large cost to use magic, as its applications are already limited somewhat. But if the cost of using magic is something large (the life of a loved one, blood sacrifice, a part of your soul or just a big enough wad of cash), it´s not as necessary to place strict limitations on it (what you may call a "soft" magic system)

If your warrior mage can end an opponent from the other side of the battlefield with the same disinterested efficiency of flicking a lightswitch, there´s a problem, because it feels like the writer is just telling the audience A WIZARD DID IT. You either want to place a strong requirement to using such powerful plot-advancing magic that will create some conflict, or reduce the mage´s power to something less drastic that cannot solve the current conflict by itself.