They were weak. But.... The manpower the Soviets could summon was just eventually overwhelming, combined with an early commitment to total war against Germany. And geographical and environmental (General Winter was a Russian ally; not so for the Wehrmacht) advantages. And finally, if the Germans had come as liberators rather than savage conquerors... And finally finally, strategic blunders, like not withdrawing from Stalingrad when they could, but instead fighting on and ultimately losing over 200K men and supplies there.
“The German Army in fighting Russia is like an elephant attacking a host of ants. The elephant will kill thousands, perhaps even millions, of ants, but in the end their numbers will overcome him, and he will be eaten to the bone.”
- Colonel Bernd von Kleist, from the early days of Operation Barbarossa.
Stalingrad wasn’t the breaking point. The battle of Moscow in winter 1941 was the official end of blizcrieg. And the Germans didn’t know another way to fight a war at this point.
They were only a partly mechanized military relying on 19th century logistics and simply had no chance in a big war.
The German success early in Europe was due to their brazenness and their opponents being sissies.