Take ostheer units, change their skins and rename it to "New Soviets"
Then you have a mirrored faction. Same tiers, same buildings, same units, same costs... but diferent skins
I only remember Warcraft 2 being 100% mirrored. Maybe first Command & Conquer games were too.
And Katitof is right. Non-mirrored factions means always imbalances. The good balanced games try to patch/fix all these imbalaces as soon as they are noticed.
vCoH e.g. was an incredible game, but it was terrible balancing the game, specially when Opposing Fronts appeared.
Let's hope that WFA doesn't follow that path.
And even Warcraft wasn't mirrored; the first one had little differences, like the human archers firing longer shots but weaker, while the orcs were stronger, but had to close in. The very first C&C was also not mirrored. Good old '95 times