Riflemen are mediocre and usf got no elite infantry.
Echelons are for emplacements builder and building holders.
Mortar doesnt have range and never hits its target.
So your stuck with mediocre infantry, mediocre weapon teams and captain tier sucks.
Late game tanks are just medium tanks which perform mediocre.
AND the micro required to play usf is insane!
Commanders needs to carry usf and if you pick the wrong commander you lose the game.
So either you steamroll them early with liteunant or you lose late game, going captain is suicide.
My thoughts from playing usf in 2v2.
My opinion for every USF unit :
Riflemen : Double BARs? LMG? Whatever you choose, you just destroy Axis infantry
Echelons : Equip them with bazookas and reach vet 3. They are just great!
Mortar : It's finally back on the same range as the Axis mortar (kinda)
Captain Tier : It rocks, it really rocks. Pak Howitzer for bombardment, Stuart for harrassment Captain himself helps field units faster with the "Supervise" ability
USF tanks : Yeah, they are medium, which means that they are supposed to face enemy infantry/armor with proper support. They ain't King Tiger, Elefant or Tiger VI, which can go solo and destroy 4 enemy bases. Sherman is great for anti-infantry support, while Jackson is the most reliable anti-tank the USF have (seconds comes the M10, that i personally love)
USF's elite infantry are the Rangers and the Paratroopers