The distance between where the gun makes contact with the upper body and where the firing hand contacts the weapon is absolutely critical if someone is to have optimal control of the gun.
If your experience with bullpups is 5.56 rifles, a bad fit means that the gun moves the person around more than is optimal for rapid, accurate shooting.
When we use that layout on a 12 gauge it completely fouls the body mechanics necessary to run the shotgun in a way that results in people getting their ass kicked by the weapon. Again, look at the people shooting in the TFTB video. They are getting their shit rocked every time they press the trigger, and little wonder because the length of pull forces them to have a much more bladed stance. It also forces the gun to sit right on joints, meaning if you aren't using effective recoil mitigation every shot loads directly into bits of anatomy that take enough of a beating on their own without adding to it with stupid designs.
This individual is at least 6 feet tall and look at where the gun sits on him:
It's sufficiently long that it is forcing the gun out into his shoulder joint. And that's a taller than average male. When we put the gun in the hands of a taller than average female:
It forces her into a more upright stance, puts the butt of the gun on a major joint, and the resulting ass-kicking-in-progress knocks her back to the point that she's half out of frame having started out framed up in the center. Look at how the layout of that gun forces her body around it. That shit is up on her collar bone and at the shoulder joint.
This gets old
really quick.
I've seen this over and over again in class. When a gun stretches people out like that it has the same effect as stretching somebody out when you are grappling. It minimizes their ability to exert force or control. And with shotguns, if you aren't pushing the gun around it's going to push you around.