That Hitler didn’t supply his troops with winter gear points to his thinking the invasion would be short and sweet. Yes, at some point in his thinking, he vastly underrated the Soviets. But so did Stalin. He moved critical manufacturing to east of Moscow and was bracing for defeat.
Supplies from the US and Britain saved Stalin’s bacon, and he even admitted it privately.
In fact German military is overrated and most of the time they solely blame Hitler it is covering their own incompetence.
It wasn’t Hitler who made all the wrong decisions.
As for Stalin if not for his purges I don’t think the German invasion had a chance to advance past border skirmishes.