I've played 4 games as the USA from start to finish, each time winning the game. At the earliest in 1945, at the latest in 1949 (with Germany turned into Fallout Reenactment Zone).
Here are some issues I noticed that make playing the US really not as fun as it should be.
1) Passive, weak and stupid AI Britain. The British are really, really important, because Britain lifted the weight of the war alone for a year irl and was able to stave off the Germans. Meanwhile, BlackIce Britain's AI is not contesting the waters of the Atlantic against Germany, submarines are not being hunted and Allied convoys are being lost en-masse, contributing to the typical horrid British casualties (800k by mid-1942? Jesus!). The Atlantic German submarines being allowed to just sail out there, free of British naval activity, means I, as the US, cannot ship my armies to Britain for D-Day without losing 200k men.
It's also not contesting the air effectively past the fall of France.
Last but not least, Britain is not fighting Italy in Africa as well as it should, in every single game British AI just loses El-Alamein and gets stuck around it, standing there for the entire game, 4+ years of stagnant front in Egypt, usually until I defeat Japan and my airforce is shifted to Europe.
2) The allied D-Day focus minitree on returning to Europe has the focuses lasting too long (50 days each is ridiculous) and their requirements are just ridiculous. The AI is not smart enough to shift the precise required amount of certain ships to certain regions, and so the entire debacle lasts way too long, well into 1945.
3) I have talked about German subs already - they're utterly overpowered. Impossible to hunt down and kill even with the best sonars. Planes also don't seems to hurt them, even with multiple naval patrol and naval bombing wings + escort task forces roaming the area.

I was able to win every time, but it honestly is a headache, because you as the US have to babysit the utterly stupid British AI. The Allies irl were in Rome by 1944, in BlackIce the British are f*cking up the Mediterranean Theatre so bad that Italy, in each game, was always the second to fall, after Germany.