It is true that teutons are better in the early game. Especially at farming.
But the farming advantage disappears later, since the teuton farm unit are quite slow (infantry or slow cavalry) - so romans and gauls get a much larger advantage out of tournament squares.
In the later game, the cheap units are also compensated by smaller crop consumption.
So after the first month or two, there is just not that big of a difference. Just different strengths and weaknesses.