It would end up being overpowered if it were a one hit kill on core. However, as it stands, it's almost useless offensively. The lock on and lunge system of the shield is terrible. It will often refuse to lunge at all, and when it does, it's usually past the enemy. Because of the low movement speed, when you do hit someone the first time they usually move around your shield and spray you in the side/back before you can even start a second hit. And in the event that you do manage that second hit, you again run into the problem of whether or not your shield is actually going to hit them. The MW2 and MW3 shields were fine offensively. This one fails at offense and defense if you're actually holding it. As a portable head glitch it works well, but then you're not using the riot shield, you're just using a head glitch.
Unfortunately they'll likely never fix the lock on/lunge, and considering the popularity of it among "pro" players, I doubt they'll end up nerfing its durability when in portable head glitch mode either.