Grens, Volks and weapon teams rely on cover and positioning in order to compete with soviet and USF infantry early game except for conscripts. Both is denied by flamer penals, mortar. So you're loosing the early game even harder. Shortly after you're looking at guards with lmgs and thompsoned-up rangers. Both axis factions have to commit at least two units to combat a single allied unit of any sort. Even though guards, rifles, penals, Infantry sections... are slightly more costly that just doesn't play out overall. On top of that you have no chance to survive without some sort of AT after like 10 minutes to repell t70s, stuarts, AECs.
Why don't you post a replay of your own where you're struggling as USF?
I remember OP Obertruppen + Fallschirmjaegers...What the hell...+Infrared+Stuka+KT = OP