There will be two parts to this review. The first will be regarding the gameplay elements. The second will be about the historical accuracy (or lack there of) and plausibility.
First off, the gameplay. At release, Sunset Invasion was essentially a remodeled Mongolian invasion from the west. A novel idea, and it made playing as the western nations a much less 'safe' option. While this was all well and good, at the time I would say that the DLC was very much not worth the attention of any serious Crusader Kings 2 player. It provided little in the way of gameplay improvements, and was one of the few DLCs ever released by Paradox that could be considered 'optional'.
As time has gone on, however, and with the release of Old Gods and Charlemagne, it has become a much more interesting DLC. Unfortunately, few people realize the potential of this DLC, and fewer still realize that Paradox has been slowly improving it, balancing it, and adding features to the Aztec that are both unique and interesting. Most of these features come about when you switch countries midway through, and creates very interesting multiplayer situations if you're so inclined. This is for advanced players, and if you just got into Crusader Kings 2, I would say avoid this DLC, or at least avoid activating it until you're acclimated with the core gameplay.
For the second part of the review, we have plausibility, and accuracy. Let's get accuracy out of the way, for those of you that didn't graduate elementary school: The DLC isn't historically accurate. At all. Onto plausibility. It may surprise many Europeans that at the time of the original start date in the 1100s, the Aztec culture and technology was equivilant, and in fact surpassed their European counterparts of the time in some regards. This is not to say that the Aztecs were more advanced, but they were far more advanced than most Europeans and Americans of European descent assume. They had a stable road system, large scale agriculture, advanced architecture, a written language, and high level philosophical ideals. During this time period, and during the time period known to Europeans as the Dark Ages, the Aztecs thrived, and were in fact in what could be considered a golden age. When Europeans discovered and subsequently usurped Aztec land, they were in the middle of their own, unique Dark Ages. Tribal behaviour was on the rise, city living became less and less common, the road systems were in disrepair, and their government had all but collapsed under the weight of social and environmental disasters. These are numerous and quite fascinating, and I recommend looking into it more if you'd like to get a bit of a wake up call on the so called savages of the Americas.
Now, with all that said, is the premise of this mod plausible? Yes, and no. It requires a chain of events to happen that in themselves are not plausible, but if assumed to have happened, makes the scenario itself plausible. Let me state that in more detail.
This DLC would require a multitude of assumptions to become plausible. First, that the Vikings made it to the Americas. Secondly, that they made it far enough south to encounter the Aztecs. Third, that when the Vikings encountered the Aztecs, there was some level of communication between the two civilizations. Fourth, that this communication resulted in detailed map, technology and culture trading that would be uncharteristic of both civilizations around that time. Fifth, that the Aztecs found just cause to use this technology to take over large swaths of land within the Americas before setting their sights elsewhere. Lastly, that the Aztecs, with the infinite amount of wealth and land that they have after conquering the Americas would then set their sights, for whatever reason, towards a mythical land to the east.
All of these individually are improbable, besides perhaps number one. However, if the question is, within the constraints of the scenario laid out here, COULD the Aztecs successfully create a fleet large enough, supplied enough, and manned enough to participate in the events that unfold in Sunset Invasion? Yes, they could. That would be 'plausible' within these constraints. BUT, I restate that the events themselves are so absurdly unlikely as to require you to suspend a large amount of your working mind to fully believe this is a plausible scenario.
In the end, it all comes down to do I recommend this game or not? Yes, I do. I find the concept interesting, and if taken as almost low fantasy, it does a wonderful job of making an interesting and unique world that is entirely seperate from the one we live in today. There's a certain level of joy in that, and it's backed up by now solid gameplay both against, and AS the Aztecs, something that I would not say in the base game. Should you get it? If you own Charlemagne, the Old Gods, and Republic? Yes, definitely. Otherwise, hold off for now until you're deeper into the Crusader Kings 2 experience.