Overstacking might actually be good.

As I understand it, when more than 10 units are stacked, only the 10 most effective units can give damage. All of them recieve damage, and fairly evenly if they're the same armour class. "Overstacking" or stacking beyond the optimum state-based damage efficiency, is generally frowned upon.

However, wouldn't stacking multiple units be effective? If I stack 10 anti-tank, 10 anti-air, and 10 infantry, it's nigh invulnerable to infantry, tanks, and planes each, as the maximum number of counter-units for each of those is present. Granted it'll affect the speed but sometimes that's not important.