You have to keep in mind that WWII was a time with an incredible speed of innovation in technology and warfare.
My point is, I wouldn't count on the industrial capacity of the countries inbetween Germany and the USSR, they were too far behind in technology and infrastructure. For example, Poland used horses for transport not vehicles for their army, so they won't have much of a production capacity in vehicles even after the germans occupied their territory. You can't build up factories and production lines with all the required infrastructure behind it in occupied territory and during wartime in only six years the second world war lasted, this is simply not credible.
Still, I would rather have a game which is not completely historically accurate but balanced and fun to play, which is what Pdx is trying to do, I believe.