As mentioned above, the issue isn't being the best because as you already mentioned it, USF riflemen are already superior to volks and, on the other hand, need to have a certain balance with Pzgren and Obers.
The problem is more about timing and manpower available to outspam the USF on the early game disponible for OKW.
Actually being the more versatile infantry is part of the issue. If other faction have equally or better units the faction design issues kick in.
It is the same issue with Wer mortar, it was superior due to faction design, and similar weapon in the hands of the USF was proven to be simply broken.
VG's ended up losing their trade mark vet 5 veterancy bonuses because of the versatility the ST44 upgrade provided.
Either all faction will become similar with similar tech and similar unit or factions will retain their uniqueness both in units and design.
Being the most versatile mainline infantry is a USF trade mark and that is why it comes hand to hand with limited access to support weapon via tech tree.