Sadly the most balanced RTS games with mirror balance are most boring ones.
Planetary annihilation anyone?
Cossacks?
Warlords Battlecry?
Both mirrored for about 80% yet the best RTS ever. Especially Cossacks (still has bigger playbase than CoH2
).
I would add even Battle for Middle earth since there are many mirrored things there.
By mirror design I mean:
Axis have non-doc LMG? Allies should have as well.
Axis have insane TD? Allies should have.
Allies have close-combat unit? Axis should have
etc