It's better to have one supersoldier/war machine with the power of a billion soldiers than to actually have a billion soldiers. The giant force will eventually weaken as people get killed, but the supersoldier will have maximum damage for almost the entire time, amirite?

For the sake of simplicity, we'll assign MMO-like stats to each. A bullet deals 10 damage and an explosive 100 damage. A soldier has 5 defense and the supersoldier has 101 defense. The soldiers will slowly die, but no weapon can inflict damage on the supersoldier, so he's invincible, although more damage than his HP was supposed to be done in total.

There's a problem when you want to split the force to defend the base, but if you have that kind of supersoldier, you probably also have automated turrets.