Played through the Union in its entirety, and TBH you have to game the system in a lot of the maps. Especially Fredericksburg and Marye's Heights. It is impossible to take the heights. And, if by some act of god you do, you will not be able to continue after Fredericksburg because of the losses you took. I kept my entire army bottled up on the bottom corner of the map, let the clock wind down and then sprinted to capture Telegraph Road while I finished up at the bottom.
South Mountain was an absolute mess as well. But it is possible to win every map at least as union.