The armour is not poorly constructed. The real protection is the underlayer which spreads out kinetic energy. That being said could Shin break Metz' sternum? Maybe, maybe not. I suspect the blow most probably hurt. Possibly cracking or fracturing ribs. I leave that to Melon. 


The real issue the entanglement that follows. (On my phone since I am out. I will continue with an edit once I reread actions)

Metz has a knife, he collided with Shin's fist. While the blow hurts it will not stop Metz from stabbing Shin. (Will do more getting ready to eat lunch)