For instance - say that your opponent FFs with Spain.
If you face Falconets you need Arrow knights. But if you build Arrow knights and he does not build Falcs you end up with a unit that is almost useless in the first battle. Compare that with the European civ that answers Falconets with Falconets – even if your opponent does not build Falcs, your own Falcs will be very useful in the battle.
Say your opponent goes Skirm + Lancers. Then you need Manchulatins + Eagle knights. But if you guess wrong and he goes Rodeleros + Lancers your army is screwed since Manchulatins does not counter Rodeleros and Rodeleros do well vs. Eagle Knights.
But if you go Jaguar Knight + Manchulatins in anticipation of Rods + Lancers and he instead goes Skirms + Lancers you are screwed again since he can easily micro your Jaguar Knights to death but you cannot do they same vs. Lancers due to Jaguars Knights being too slow.
Compared that to Skirm + Dragoons + Falcs which does ok vs. any army or even better Janissaries + Abus or Forest Prowlers + Mantlets. Aztecs are simply more vulnerable to the wrong unit choice then other civs.
[This message has been edited by MockHamill (edited 02-02-2007 @ 05:01 AM).]