What they are saying is true. It is very deep. I have only scratched the surface, but Zombies is a big indication of the development that went into the game. Zombies is Zombies, but there is a metric shit ton more to it. I dont want to spoil it for anyone, because when I experienced it I was like WTF? and you should have that moment as well.
The Campaign mode allows co-op and the game borrows from some successful entities in gaming and movies. I am at a point in the campaign where I am experimenting with Force like powers but think more Neo than Anakin in there application. Pretty cool stuff.
Online is also intense. You can run along the walls now and the maps are designed for you to take advantage of this and also engage in firefights whilst running along said walls. "Exosuit" has been tuned down, and Destiny type specializations have been implemented. You can select a player specialization that will give you a choice of two abilities that charge over time. Abilities like being able to warp time backwards for a brief moment, see enemies through walls, pull out a semi-automatic grenade launcher, etc. Its fast paced, like always.
More mXm on any game is always a good thing.