It does seem to be becoming a selling point, eh?
Though the USA isn't comprised of loyal skaven, per se. Their training regimen is harsh enough that it basically gives them institutionalized PTSD at the thought of betraying their commanding officer, which is why Krasskik and Skrazz took Sleek to get help rather than leaving him to die despite the fact that he took a page out of Pai Mei's training guide. Outside of combat though, they'll happily advantage themselves over their compatriots as much as they can get away with. What really unites them is ... basically proto-patriotism. They, unlike pretty much any other skaven in the Under-Empire, actually believe in the overarching ideal of the Under-Empire, and have something resembling pride to serve as its army. Other skaven have their loyalties to themselves first, themselves second, themselves third, and fuck those guys from other clans fourth. USA skaven are mostly the same, but they see themselves as part of the Under-Empire, something bigger than themselves which includes all skaven. So they seek to benefit themselves and the under-empire in the same measure.
Of course this is mostly implanted by drilling so vicious it's basically regimented torture, but it works so who cares, right?