Um, The main problem for the Aztecs was lack of metals, (except gold and silver) especially iron. Being that they lacked any significant iron, they did not develop the skills to work metals with high melting points. They made weapons from wood and obsidian. Now obsidian is really effective against naked skin as it is extremely sharp. The problem with obsidian is that once it meets any significant resistance it shatters and becomes useless. The Aztecs also lacked cavalry. Now clubs can be effective weapons against steel armors, but given that the Aztecs generally went virtually naked into battle... Aztec ranged weapons are also somewhat lacking. Their primary ranged weapon was the Atlatl which is sort of a spear thrower. This allows the wielder to propel small spears with decent range, power and accuracy up to about 20-25m. Given that the effective range of European missile weapons is anywhere from 100 to 400m...I would expect another huge advantage for European armies. To sum up, given the advantages of steel armor and weapons, superior ranged weapons, and most importantly cavalry, I would expect a competent European lord with 20k men, including 3k cavalry to be able to decimate an Aztec army several times more numerous.
The Aztecs would also have a great deal of trouble with cold weather. I would expect massive attrition for Aztec stacks in northern climes.