Economic reliance on having a pool of infantry to produce resources results in a lack of flexibility
With a preponderence set towards technology, research and resource-gathering, the United States is a faction that requires strong strategic flair to use. The United States shares some units with England and France, but in other areas it also has other units worth noting: a line of medium ranged and melee cavalry; a Marine unit that emerges two ages earlier than other factions'; as well as a slew differing units at differing ages. It is thus somewhat of a challenge using the United States, as one has to keep in mind that American strengths and weaknesses often shift this way and that way - but one can be assured that by the last two eras, the United States is an extremely strong military force.
Marines are the heart of the American war machine. With the ability to entrench themselves, the Marines will be useful for creating fortified positions which will be hard to beat without resorting to a flanking attack. Thus, they work well as a meat shield that can absorb plenty of damage until you can manoeuvre your medium cavalry and musketeers into position for a good attack.
Unlike all western factions, the USA has no access heavy cavalry variants, but this is very much offset by the fact that their cavalry, being medium cavalry, is highly versatile. Cuirassiers may be mighty but do not function well due to their speed and cost. Thus, when needed, mass as many medium cav units to swarm enemy cuirassiers, or break your cavalry units into individual groups for fast hammer-and-anvil strikes on unprotected targets, such as skirmishers and isolated line infantry.