Personally, I feel that there should be a distinct "Elite Assault" class put into the game which pretty much has the option of MP40/MKB42 for Axis & PPSH/AVT-40 for Allies. Furthermore, the "Elite Rifleman" should have access to the Bolt/Semi-auto rifles for Axis & Allies. Riflemen/Assault should be restricted to the Bolt-Actions/SMGs as they aren't supposed to be specialized. Squad Leaders/Commanders can probably remain with the Semi-auto rifle (as it is conducive to non-hyper aggressive play which is currently present in-game) but not have access to the AVT-40/MKB42. In my opinion, the Axis decision for what assault troopers use now is pretty much a no contest issue in favor of the MKB42 whereas with the Allies, there are definite trade-offs between the PPSH / AVT-40.
I'll admit, I'm pretty good with the MP40 but at longer ranges (150-200m+) you generally need to do a number of short bursts to potentially even score a hit, let alone a kill. With the MKB42, if someone isn't moving it's a 1-2 shot kill with it on semi-auto selective fire mode. For Axis, it's pretty much a no-brainer to pick the MKB42 because in almost every category, it excels far beyond what the MP40 is capable of and starts to compete with rifles at medium/long range with accuracy/kill capability. The only thing I might even be able to say against the MKB42 is that it feels like my stamina bar runs down faster with it equipped, but that could very well be incorrect.
Conversely, the AVT-40's full-auto capability "can" be useful at times, but largely it suffers from far too much recoil even when prone/with the weapon rested. The PPSH is not too much different with recoil, but generally speaking it is more prudent to use the PPSH over the AVT-40 for actual assaults (moving into close quarters combat/clearing buildings) as the sheer number of bullets is far more controllable unsupported than with the AVT-40. Furthermore the PPSH, despite it's high fire rate, can be used effectively against more targets whereas with the AVT-40 it generally suffers from expending it's clip exceptionally fast (though it will likely kill anything directly in its sights). There are trade-offs between these two weapons whereas with the MP40/MKB42, it's pretty much a "be smart, use the MKB42" situation. Another point of note is that the AVT-40 in semi-auto selective fire mode basically becomes a reasonably accurate semi-auto rifle providing an assault trooper with excellent medium/reasonable long range killing power.