I find that balancing the magic in a game can almost the hardest task there is.
If you have ordinary non magic units versus magic units and magic is too strong non magic units will be largely bypassed and vice versa.
Balancing those for 1v1 and team versus team is also a very different kettle of fish.
Magic usually has a micro cost and the use or non use of that micro time is affected by range, when terrain and distance enable the use of magic it can be devastating and sometimes unfair in the mind of a player.
If you make the spell too powerful and wide ranging (splash damge or area of effect) then the player with many non magic units may feel that his time and efforts have been cheated, akin to the "Instant Death" that RPG players hate.
In TA Kingdoms to see a almost complete standing army decimated by a single spell casting unit is ...well definitely not fun. In TZAR to see a unit with Rain of Fire spell book arrive and take out a standing army or even a platoon almost before you know it is also well up to argument as to whether it is a balanced.
Personally I think that magic units are such subjective balance issues that AOM may have opened a very ugly can of worms when they release the product into the hands of the player community. I think that magic is better too weak than too strong perhaps akin to the nudge on a pinball machine there to tip the scales not to dominate at the expense of 'traditional units'.
Range... is a balance issue in itself how much is range worth and how does terrain protect the magic casting unit?
IMHO