Quit being a smart @$$ M, any faction can be balanced, Suppose modern has lots of fast, ranged attacks, with a few muggers or something attacking with knives, and cavemen could be more powerful, but slower attacks.
Last option, give them all the same power, but different looks. More boring and not reccomended. My point is, how balanced they are depends on the stats, even if in reality they would be unbalanced. This is a fantasy game. Things are not the same as reality. (I don't seem my neighbors casting spells at me at 2AM, do you?!?)
Think of it, my work-in-progress ancient tech tree has four factions, (hellfire, omega, twilight, and church) Omega sounds the most powerful, and has a unit called (ironically) Omega which has 5000HP, but I can STILL beat any faction with any other faction. Why is that? Because I balanced them properly!
SHEESH!