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Just a whole ton of animation work would be involved.
How much animation work, exactly? Kind of a brag, but I was able to create animations for pistol dual wielding mod for XCOM 2 in three weeks or so. And I'm an amateur.
A lot of animation work. It is a new wield type so there would need to be custom locomotion animations (passive, active and ADS stances, as well as jumps, falls, and lands), reloads, deaths, firing recoils, additives for aiming, custom vehicle animations, hit recoils, etc. And that is just the actual animation work. There also needs to be the animation network support for handling all of these new animations. It also has to be done for both first and third person for everything I just listed. Looking at the video I am sure you have a sense of this stuff, it is just a in a larger scale than XCOM. It is quite a time investment to do this right.