Due to the relatively short amount of time at the front, and particularly because of the constant influx of replacements, regular US infantry units never got quite as seasoned as those of some other countries that lacked the American manpower, training and organisation.
But once they did, they became the iconic imagery for the allies - the 'soldier to be'. Determined and proud, they fought on in any situation, often to the last bullet.
Tough as nails, you can give these vets any task and they'll be able to do it. They are not invincible so make sure they are supported, or advancing behind some form of cover. Better yet, give them some transport and get them to the objective asap!