I remember this discussion going on after the first patch. Some of the best French players were whining that they could no longer beat good US players beacuse Doughboys were made cheaper and, IIRC, faster to produce. Hellbringer even posted a game in which he rushed a (very good) US player and in fact, he did lose the game. What that game demonstrated is the fact that US has to execute a
perfect build order, getting out the first DB in 3 minutes or so, sacrificing some econ to get the barracks up, a house for morale nearby and the first DB out in that short time-frame. Even so, that game could've gone either way for a while.US is definitely vulnerable early, but if they survive that initial rush with great micro and get their econ going, they can be very tough to beat by the 10 min. mark, even with an all-DB army from double raxing. But when you reach 20 mins. and you've got air attacking US units without mobile AA, it becomes very difficult to attack as US without a lot of casualties, assuming the opponent goes to bombers. Having no defense other than (not particularly great) US fighters adds a lot of additional micro (fighters + dodging bombs) and cost (US fighters are expensive for what they do) for US offensives.
Overall, I'd say Germany is harder to play than US, simply because you're always at a serious numbers disadvantage in the early game, and you can't quickly diversify your units to counter enemies that can shift quickly if they want to force a certain style of play on the German player. You can almost always spam the type of unit the German player can't counter readily with enough units. And then the German player has to expand rapidly to maintain enough gold mines to support its needs, and has to do so with fewer units. That's a very tough combination.
Then there's the UK. Fewer people played UK competitively than any other, and that was because they probably sucked worse than Germany unless they villie rushed, in which case they pwned everyone. Honestly, Empires would have been a great game if there had been a little built in buffer for the slower civs. In the early development of the game, there was a feature called "State of Peace" which allowed you to set a minimum time-frame for attacking (a truce, essentially). They took it out for a bunch of reasons, but the idea was essentially a good one. I would say a 7 or 8 minute buffer to start any game would be enough to give every civ a reasonable chance.