why are german machine guns firepower 4 and americans 3? seems unbalanced as the germans have better tanks too

This is because the German MG42 (and even the earlier MG34) as well as their tanks were the best in their class in WW2 period.

Most of the scenarios are simulations of events, so balance per se is not present. Look for internet comments on the scenarios (like on boardgamegeek) to know which ones are most balanced.

In BGG, there are also player submitted content to the ToI BGG site on a point buy system. Sean Doubt made one, and I've collaborated with him and modified his system a bit. You can look into it and try it to see if you can build lists which balance out, since the German MG plus their tanks are more expensive than the allied and russian counterparts…

edited: double post