Both of the Red Alert 2 sides are pretty well-balanced in my opinion.  But the Allies have some very useful items that give them the edge over everyone else.  The first advantage that they have is the GI, it is a lot more powerful than the Soviet Conscript when in its fortified position.  Another advantage is the rocketeer which, when in numbers, can deplete the threat of Kirov Airships completely.  Another good unit of the Allies is the Spy, with this unit you won't have to worry about money anymore.  Another big difference between the Allies and the Soviets is that the Allies have an air force.  The Night Hawk Transport and the Harrier are very useful.  Another important vehicle that is good for the Allies is the Prism Tank, they are extremelly effective agaist all buildings.  The last vehicle that is the advantage over the Soviets is the Chrono Miner, it can gather up money a lot faster than the War Miner.    On of the best structures in the game, in my opinion belongs to the Allies, it is the Spy Satellite Uplink.  It can completely remove the shroud showing the other teams bases.  The last useful structure is the Weather Control Device.  It is good because it lasts for a long time which will do more damage.

The Allies do, in fact have some disadvantages.  One great disadvantage is that they don't have vehicles with armor.  Their tanks will get destroyed too fast and too easily.  Another disadvantage is that the Prism Towers will always come together to form one beam.  So they can only destroy one enemy at a time.