The problem is Riflemen facing Volks + Sturms while RE are horrible in the first minutes. If it was only Riflemen vs Volks then USF would have an easy time.
Even that is not really the biggest problem I think, because you know the Sturmpioneers will be coming so you can team up your RET and RM or wait for your second RM before pushing. The bigger problem is that the second and third (and fourth) Volks come out a lot faster than the second (and third) Riflemen (or officer) and it's easy to get overrun because stock Riflemen are not guaranteed to trade well (even though they can). The OKW bumrush will hopefully be addressed with some faction overhaul changes in the upcoming balance patch, by toning down their early game power and spreading it out a bit more across the mid game.