The different approach in ZXMOD is that it's based on the "small is good" philosophy.
This means that rather than trying to spice up the game by adding dozens of units, the mod gives a more focused and specialized experience by restricting the unit roster available to any nation and giving emphasys on their role and national units.

You can find a more detailed explanation in the previous beta thread: https://forums.totalwar.org/vb/showthread.php?t=48703

Regarding the "new" units from VI, It's simply the use of part of VI units in MTW (the mod requires VI BTW) in what I feel is a logical and gameplay enhanching way.

Regarding crusades, though more factions can use them they aren't a really cost effective solution now, as the price is doubled and the "extended" roster means that you're more likely to get lighter units than knights, especially in early where you can end with a bunch of peasants.

Finally, this MOD isn't meant to compete with the big ones such as XL,BKB or MedMod, but rather to give a quick and refreshing experience to the player without having to learn everything again as the focus is more on gameplay and AI improvement than a true "novelty" factor.