Poorly designed and will break, not to mention the way they are strapped to the arm may injure the wearer during use. The reason blades as weapons continue to be handheld is the infinite tactile variations upon brandishing said weapon.

Wrist mounted blades may LOOK cool but are highly impractical as true weapons, they cannot be flipped around from stab to feint handholds, they cannot be switched from hand to hand, they cannot be thrown, they cannot be adjusted on the fly in really any way shape or form.