The problem are the Victory conditions. In "Road to Leningrand", the German must conquer Leningrad to win, OK. In "Road to Moscow, to win, the German must conquer Moscow", OK. But in GC 41 the German must conquer not only Moscow, Leningrad, Kharkov and Rostov, but too Stalingrad, Gorky, Tamboy, Saratov, Stalingrad.. ¡and even Baku!
Simply is not possible against an human (an human who knows how to play, of course).
The soviet, to reach a total win, only needs to conquer Berlin before 1945, is difficult but is possible.
It´s like playing chess with this rule: white side, to win, must kill the black king. Black side, to win, must kill the white king, and the queen, and the horses, and...
Who wrote the Victory conditions? Stalin?
And the fall of key cities as Leningrad, Moscow and Kharkov, means nothing in the war, there are not moral nor administrative penalities, only a little manpower and rail capacity less.
To balance a game, you can modify the history (doing stronger or weaker a side, that historically). I don´t like this, I like history even unbalanced. Or you can have a Victory conditions that will give to both sides the same chances of victory, more or less.