I've always imagined that without the bit of machinery made specifically to channel/create aura ala what Penny can do, his Aura's protective ability would only be limited to the fleshy bits about him, which are really just a few organs and most of his brain. It's thanks to that machine however that he can channel aura's protective qualities throughout his entire body, though he still layers armor on himself as a contingency if his aura fails/doesn't need it for whatever reason. Related to that, I mentioned before that Gren's Semblance has a hard time "moving" through multiple objects I.e. going across the floor is easy enough, but making it go across the floor, onto a chair, and then the person standing on the chair requires more energy then usual. Even just being on a higher elevation is more exhausting for Gren to do then simply being on the same location. I've always imagined that this is due to the fact his body is mostly mechanical, as his semblance first begins from his fleshy bits, and then has to travel throughout the complicated machinery of his body and get to the ground, which is then redirected towards whatever he's aiming for at the time.

Basically, while being a cyborg has had no notable effect on his aura, if he was fully flesh-and-blood his semblance would be much more powerful. We know Penny is capable of producing aura, but what we haven't figured out yet is if she could manifest a semblance. Presumably yes, but nothing conclusive. So I suppose another weakness to bricking his semblance would simply be standing on something while you keep moving.