Due to how Conquest generally works and how as you progress, you begin to unlock more technological and technical levels, it makes US Army the best faction in Conquest or anything late game.

If you play Skirmishes like Domination, Combined Arms, US Army is nowhere as OP as it is in Conquest due to some weaknesses such as lack of early enough source of heavy anti tank and OPFOR light vehicle spam early game that can be truly threatening to US. The US lacks any early to mid game MBT or medium tank as well.

However, it is compensated by access to Javelin teams or the Javelins inside their APCs and IFVs. US infantry squads also tend to have good firepower at close-medium range.

Other factions have their own perks that can be better than their US counterparts such as Germany having better mechanized options and vehicles, as well as mid game MBT and stronger conventional anti-tank launchers. OPFOR being able to have tanks and light vehicles earlier than US.

But overall? US Army is the all-around best faction in most circumstances. It's easy to play and the best faction overall.