Well, nobody's perfect, I guess everyone had a few successes or defeats here and there but if you try to find an average success rating I would think there are quite a few differences and then you try to find out what these differences were. Just looking at the casualty figures of the war one could say the soviet success had something to do with manpower, they lost so many, yet still outnumbered the german forces heavily. I think the more you outnumber an enemy the less losses you take might also apply since you have a lot more firepower and even if you can't hit a thing the enemy will be extremely disturbed by the amount of lead and explosives you throw into their general direction. That doesn't mean the soviets couldn't hit a thing, they even had the best snipers overall, I just think at some point it becomes hard for the defenders to stop the mighty steamroll.