Three things:
1.)
Again, the Soviets and Germans had roughly the same amount of SMG's pr. soldier, and as such giving both teams the same number of assault spots is realistic.
Keep in mind that the Soviet army employed 5-6 times the amount of men that the German army did during the war, and as such would need 5-6 times as many SMG's to keep the same percentage of men equipped with an SMG.
While this is true of the war effort in general and the relative size of the armies, it does not hold true for the battle of stalingrad in particular. At the start of the offensive, the german attack numbered roughly 275000 men, where as the russian forces numbered slightly below 200000. A ratio which actually favors the germans. The only time this skewed the opposite way was was during the soviet counteroffensive and operation uranus the following spring which employed 600000-700000 german forces vs around 1100000-1200000 soviet, which while going on 2:1 is nowhere near 5-6x.
2.)Not that i believe that the soviets need more assault slots, they do not. I think limiting these roles as it is now is a step in the right direction and habing them equal, while not necessarily historically accurate, will keep the game more balanced and more fun to play, especially as a boltie regardless of which side you play. As it stands there are still plenty more SMG roles in play, especially on maps involving sappers or engineers.
3.) Regarding PPSh accuracy. While it is understandable that many people take the PPSh to be some kind of bullet hosing automatic shotgun for idiots manufactured out of spare parts, the reality is far from the truth. At its core the PPSh is actually a very elegant and effective weapon that has more in common with the modern FN P-90 SMG rather than the uzi.
It has a fairly long barrel for a weapon of its size/caliber and fires a necked 7.62 cartridge at significantly higher muzzle velocity than the comparative 9mm parabellum of the MP-40. This actually results in higher accuracy at long range purely from a ballistics perspective as the lighter, faster bullet will have a shallower trajectory.
Are there other factors? Yes, we must of course consider tolerances and limitations of wartime production. A gun with loose tolerances and a poorly manufactured barrel will shoot worse than one that snaps together tighter than a nun's legs and was made in a laboratory. However, just because a number of PPSh's were put together by 6 year olds out of recycled Mosin-Nagants does not mean they all were, and it was fairly far from the norm. Incidentally i do not see how using a Mosin-Nagant barrel designed for a more powerful round and a much more accurate weapon could possibly hurt the SMGs accuracy.
Now while yes, the average quality of manufacture of all PPSh SMGs is probably lower than the average qualiy of all MP-40s, it is just that, an average. Very well made examples of PPShs exist, and existed during the war, especially ones of pre- and early war manufacture. Thus it should not be impossible to find an accurate PPSh just like it is not impossible to find an inaccurate MP-40. And we do technically have this represented ingame as the 'weapon qualiy' stat.
I just do not see why people keep insisting on treating any 150+ meter kill with a PPSh as some stroke of god.