Dafuq you mean "come back mechanics", like It's some sort of thing Relic put in the game, comebacks are a result of smart play and perserverance, I've lost to people whom pushed me back and won after I controlled the entire map and they had only 3 VP's left. I've done the same aswell. And you don't even answer the question, you just say give up and move on.
So, to answer the OP's question as USF. USF tends to maintain a more mobile and faster force, mitigating that for tanky units and brawlers. If you have tactical support, Halftrack with AA and calliope are fairly decent for pushing back. With heavy cav, pershing if you can hold on. Armour company is really dedicated to pushing the enemy back, the M10 and Bulldozer and really great units at what they do, if you can't gain a decisive advantage, select the Armour company.
Basically, flanking and Premium commanders. That's just how Relic has made the USF. Kinda linear, but it is what it is.
There is a comeback mechanic, its called upkeep cost, the other one is popcap. Without these army of winning player would grow so fast that comeback would be impossible.
@OP: there is a comeback chance for usf, the calliope, just like any other rocket artillery it bleeds your opponent so you can comeback more easily. Still its hard, especially against okw who have far superior late game.