I think that Bombers are extremely over powered. Every game is a beeline to flight and once there, building anything but bombers a bad idea. All you need is bombers and cavalry to take the cities. Triplanes are garbage at being a counter and terrible at fighting themselves.
However this is realistic. Flight changed the way the world fought wars. Air superiority dominates any war/battle. 1 bombing can do more damage than an entire platoon of ground troops in reality. Imagine if some one tried to invade the US with no form of anti-air. They would get wiped off the planet with almost no losses to the US. Everything they have would get destroyed before even reaching US soil.
CraigMak, I agree that it's realistic now, but it's not realistic during the era in question. Bombers did not play a huge role in deciding the Great War. And as has been mentioned, gameplay-wise it's less fun when the proper strategy is to spam a single unit that has no counter. I honestly think the GWB needs to go completely, both for historical and gameplay reasons.