If you manage to keep your vet3 BAR/1919 Rifles alive and are halfway decent at microing two Jacksons, then there's nothing you can't take on late game.
I seriously don't see how Rifles don't scale well late game? I mean you have nothing to spend your muni on except the guns from the weapon racks. Their 5 man squadsize is optimal imo, you fit in pretty much every cover but still have that special dude that always survives the nade/tankshell/whatever explosive.
As for the lategame sacling of grenadiers, their low squadsize makes them disappear pretty quickly late game, when the heavy hitters start rolling out.
As for the USF indirect fire. Yes the Mortar Halftrack needs a buff, but the pack howie is nothing short of awesome. I build it pretty much every game and it rarely ever finishes the game without vet2.
Edit: This is from a 2v2 perspective.
They doesn't scale well because their vet 3 only give them more damage output instead of accuracy like most grens, cons, and volks. It's reasonable since rifle has semi-auto rifle, but it makes rifle suffer the late game since most fight will took at long range and without much cover (mostly yellow or even none).
And their vet2 doesn't increase their survivability as much as grenadier's vet 3 does, they are still fragile as fk.
I would say rifleman scaled pretty well but definitely no better than OST and OKW.