Soviets were vastly undertrained and had a very depleted officer corps as a result of the purges.
Even though units might have had modern equipment, the few officers around were terrified to train with it as any major breakdowns or losses could result in a trial for sabotage.
That applies to early war, not late.
Late war soviet army was full of experienced veterans.
That's why everyone considers it pretty damn stupid that we have conscripts in multiplayer, which happens in 44-45 years despite there being strelky in game(soviet campaign).