I am really enjoying a somewhat historical Japanese playtrough, and so far (i am in mid 1942) i have noticed two issues with the USA AI:
1. The US never built a single battleship, they entered the war with the same ones in service in 1936, which seems a bit unrealistic. Also the british did the same, never building a single battleship.
2. I have noticed that by spring 1942 the US AI has gone into "national defense" mode, building mainly land and air units, and stopping building naval units, except a couple of destroyers and sometimes a carrier- no subs, no cruisers, no BBs (howewer it started the construction of several CVs at the onset of the war). To me this seems reasonable if the US mainland was in danger of invasion, but so far i have made realistic conquest in the Pacific, taking (from US owned territories) Guam, Wake and the Aleutinian Islands - not even Midway nor Pearl Harbour.
I have had a look at the ai event file, and the US AI is set to go into "national defense mode" when loses 2% of its national provinces. I think this treshold should be way higher, something like 10% - or change the trigger to the US having lost control of Pearl Harbour and their other pacific bases. At the time i think it is enough for a Jap player to conquer Wake, Guam and some of the Aleutinians to make the US ai almost stop building naval units - except for some carriers, whose production is governed by another event as i see.
Finally i have noticed that by 1942 the US are bit too much behind in aircraft, subs and capital ships techs, still building Yorktown class CVs, 1936 submarines and biplane bombers. This was also noted in several other playtroughs.