An axis player in our last game tried this very strategy. It was clever. Build factory complexes in Romania and Greece to transport units to the south of Russia by sea. It did not work.
It does not really matter from the Russian point of view if the enemy come from the south via transport or walks across from Romania. They are both about the same distance from Moscow where almost all of the Russian units are produced. So the infantry and artillery hordes still have to be beaten.
I played Russia and he transported about 12 units south of Stalingrad. The axis had to retreat when about 20 Russian ground units showed up.
Do not mistake me. I am not saying an offensive toward the southern provinces where the money is will not work. It just cannot be done on the cheap. Somehow, someway the axis must possess a bigger army than Russia to drive her back.
I think the money spent on navy (to protect the transports) is a waste in most games. In the few games where the axis has a big enough army to hold the Russians down and money to buy navy to go for a southern flanking move, they could just as well buy armored units to do the same thing and still use the armor in an attack on Moscow.