Crusader Kings II: Sunset Invasion Review (kyberus)
In terms of gameplay, the Aztec invasion is interesting once and only once. If you are starting significantly earlier, they can make an interesting late to end game, with the sudden addition of a massive, nigh unbeatable foe. On repeat play they are substantially less interesting. They make most of the small powers of the west unplayable mid-to-late, as they simply turn up and squish you with 200 000 men. The expansion doesn't really add anything in terms of gameplay mechanics either, it just suddenly spawns a new, extremely powerful faction at the same time every game.
In terms of history, the aztecs didn't even have an Empire as of the stated date. At no point in their empire did they have a navy, much less a navy 500 years more advanced than anyone else in the world. Even if they acquired these things, the most likely target for invasion would have been their neighbors to the immediate north, given their proximity. Also, they somehow have cavalry. What are they riding? guinea pigs? Bison? The Sunset invasion makes as much sense as an invasion of magical space hamsters, but is substantially less fun. It isn't alternate history, so much as absurd fantasy.