The reason the USF are considered more "micro intensive" is mostly due to their armor game. The Sherman, for example, needs to be manually cycled between anti-infantry and anti-armor ammunition. The Jackson is the lowest health, highest damage tank destroyer in the game. The 57mm AT gun, while having some useful abilities is, at least in my experience, the easiest AT gun to decrew in the game.
Add to all of this that the USF has no heavy generalist tank akin to a Tiger or IS2, nor a heavily armored anti-armor vehicle like the Panther or Elefant, and you see where the USF late-game struggle begins. While the Germans and late game Soviets have some units they can send into the fray that can perform well enough on their own without needing babysitting, every USF armored unit demands near-constant management.
lol i just had that experience a few moments ago with human opponents. i tell u right now the panther is a big fucking scary monster. now i wish USF had the M26 Pershing