The mp40 does need a nerf but it has it's flaws too unlike the PPSH the MP40 recoils a lot horizontally making the sight 'shake' which IMO is a lot worse than it kicking up vertically as adjusting for vertical recoil is quite easy.
The MP40 doesn't need a nerf so much as there needs to be fewer of them around than there currently are. If you were to go back in time to Stalingrad, you'd likely see far more Germans carrying PPShs and using them in assaults than you would see them carrying MP-40s. MP-40s, in terms of overall production numbers, were outnumbered nearly 6-to-1 by PPShs, and MP40 distribution was usually delegated mostly to squad leaders and tankers.
What I do think needs changing is the recoil on the PPSh. You can fire accurately in short bursts at multiple ranges, but from what I've read and seen about the weapon, it seems like it should handle under sustained fire at least somewhat better than it does in the game.
The PPSh had the historical advantage in terms of numbers and availability, and of course, the sheer volume of fire you can lay down with the drum magazine. I wish Tripwire had made the drum magazine standard issue (Maybe with the stick magazine as an optional choice/unlock) and had not included the MP40/II, which is a ridiculously hokey attempt at balancing where no balancing is needed.