It is heavily implied that he does not truly control the magical energies. Directly after he truly gained them, it is no wonder that he was incapable of stopping the acolytes. This is further implied by one of the JoJ mailbag questions (sorry, don't remember which) in which someone said that Xerath was tampering with powers beyond his control. It was probably Ryze. Anyway, the coffin's remains limit his power. Even in the limited state, he has so little control over his powers. He can only tap into a tiny amount of said energy. His power is unfathomable, but the power he has that he is capable of controlling is relatively small.