RJ never really went into the OP defying properties of C. The fireballs are pure weaves of fire, no weave, no fire, no heat. We also do not know about the heat transferance properties of C. For all we know it could pass heat like tin foil, or be so resistant to it you could hold it in your hand while someone plays a palsma torch over it. We just don't know. Lightning might or might not be effective. C started as iron, but is it still ferrous? If not, just how perfect an insulator is it? It could be such a high value insulator that lighting will not strike it, again we just don't know.

An interesting debate might be what would happen if someone wearing C armor got hit by balefire. Balefire is pure OP that burns you out of the pattern backwards, but C is immune to the OP. Would it just wash off you, or could (theoretically) a channeler use enough OP to burn far enough back to before you put it on, or before the armor was converted from iron...

My position is simple. C armor would not be hard to make, making it would not violate any of the oaths, and C armor is substantially better than normal armor. just how much better is something we can go round and round on because only RJ could have given a definitive answer; but he never did.