Lol what? USF is a very mobile army in the game. Also, USF will need to commit to a commander doctrine in order to fight defense Wehrmacht/OKW. Other than that, USF has no indirect fire late game that is a stock unit.
Firstly i refer to turtle as for other RTS games, wich mean strong lategame defense, and probably a slow but steady map contol increase. So ost with bunker spam and elephant is able to turtle as fir USF with jacksons, riflemen and the new team weapons. Pack howie is a early long indirect fire abke to move, wich means it can deterr enemy defensive positions as a LeFh or Sexton will do. It is underperforming comoared to the lastly mentioned units but its avability and cheapness make up for it.
As fir USF mobility it doesnt mean that much if your intentions are holding ground, vps or defend. Its useful to hunt diving tanks or pickup oportunities but it does not affect the faction profile overall.
USF has an early aggresive midgame adaptative late game turtle style of play.
I hope this elaboration is useful, if you already knew this things pls ignore them