I think the only truly unbalanced map is Ger vs. Rus, only because Russia is too hard to invade in the north and also gets the southern building zone.
In all the others, a player is always capable of winning as either country. As Halt said, once you are at a certain level, it is never about simply choosing the "correct" moves, it is about countering and defeating your opponent. This may not be the case for Eng vs. Tur, but I always thought that one was pointless anyway because you don't even fight your opponent, you simply race to 18 sc's. It is more an exercise in mobility.
The advantages that some of the countries have only runs as deep as they are recognizable. For instance, where Germany out-strengthens Italy, Italy can easily outflank Germany with good mobilization. While Austria has a stronger land-access to the Balkans and the west, France can mobilize the north as well as pin down Austria's naval output in the south. The imbalance of these maps only occurs when it is two average players playing without the foresight to grasp such tactical scenarios. This is not always the case, but often times it is, particularly with the imbalance of FvA, GvI, and FGvsRT which I believe are all well-balanced maps, (if you realize that each country must be played differently, and that you must recognize and counter your opponent's strategy).
I read a good player's quote on webdip the other day that struck me, it went something like this:
"If you know your enemy, and know yourself, you will never lose in 100 battles.
If you know yourself, but do not know your enemy, you will win 1 battle, and lose 1 battle
If you do not know yourself, nor do you know your enemy, then you'll never win."
I think this holds especially true for 1-on-1 games, validating the point that Halt was making.