The power limit will vary between masters. I don't want masters who can just flat out go 'Za Worldo!" Dio style with time magic. Likewise leveling buildings is something I'd like to avoid seeing. So if you have say earth magic you can change formations around you, mend concrete, and overall control the battlefield by that standpoint but causing grand earthquakes, and such is beyond you and you'd only be able to use earth magic from a certain distance away preventing you from instant kill spiking an enemy servant or master.