USF has a huge problem late game with MP drain. When Panzerwerfer or Stuka comes into play the 280MP rifle are shut down. You can build cheap RE as replacements for weapon pick ups and cap points but they are so lame late game than why even bother. USF generally more micro intensive and almost all of their units require constant baby sitting.
I would second this, I don't typically find rifles lacking too much vs comparable infantry, but once tanks and indirect fire come with explosives that ignore RA, the cost to maintain 4-5 rifles(inluding officers in this), RE's any support weapons can be quite prohibitive.
Additioanlly when you get to late game, if you have gone say 2 officers that's 37 popcap, 6 for vet 3 Re's, 1 major 3, ambulance 3, without any support weapons that's 52 popcap. Limiting your tank pool (IK there is decrewing) and decreasing your manpower income. For example the other day i got 2 officers and 2 support weapons, then I was popcapped with 2 vehicles, a sherman/priest. It's hard to make decisive pushes with limited vehicles and even if successful reinforcing may still drain your reserves.
TLDR: Popcap sucks when you gotta spam rifles