Well I was under the impression that a unit with smgs in face slapping range should still win even with a cost difference unless it is a huge difference (pios vs rifles). Maybe that is incorrect.
Well if they are considered balanced they are extremely hard to use.
Sturmpioneers and SMG's in general are designed weirdly. Sturmpioneer's Assault Rifles are less effective than Panzergrenadier's at every range except point-blank (~5 meters), where they're superior. Submachine guns are weird because they have high flat DPS out to 10 meters, where it suddenly drops sharply.
Regardless, there was a patch where Assault Engineers were superior to Sturmpioneers, and it just lead to every USF player spamming Assault Engineers and decimating every axis early-game unit including Kubelwagens and Light Armoured Cars (due to the fact that all small arms have max DPS vs vehicles at all ranges).