I think Metallo could either be a solid villain or a weak villain.
If there was one thing I liked about MOS, it's that Snyder didn't use Kryptonite for Superman's first movie. It really would have slowed the movie down.
One the one hand, Kryptonite can be interesting, since it's merely Superman's only weakness. On the other hand, Kryptonite sucks. I think there's so many reasons why, but that debate would be endless, but just to name a few reasons why it sucks...
A rock that weakens Superman. Sometimes Kryptonite substitutes itself as the main villain which sucks.
To some extent, kryptonite was like the main villain in Superman Returns, and that's why I hated it. Lifting the Kryptonite island into space.
I mean it is a rock, as opposed to a super-villain whom we could see Superman fight. But instead, it's a green rock. I think Kryptonite can be interesting when put to good use though.
As for Metallo...I think John Corben is an interesting character, but if we saw Superman get beaten easily by Metallo for God knows how long, and to what length it will take up in the movie, then that would be annoying. Having said that, I think Metallo poses as a reasonably good threat to Superman in terms of story.
Aside that his heart is pure Kryptonite, he is also mechanical and has other gadgets which is interesting too. And seeing him upgrade himself into the giant, monster Metallo (I don't know if he's refered to that way) might be fun too.
Doing Metallo would be complex, but maybe if the writers thought hard about it and how to make it happen, then maybe they could pull it off. The bottom line however is that we don't want to see Superman get taken out so easily by Kryptonite, because, then the fight is practically over as Superman can't possibly continue the fight once he's weakened by Kryptonite, so it doesn't make much of a point.
Because how then, could we see Superman battle Metallo? But say they play it out like this...Metallo wants the fight to be fair, or he wants a good challenge, so he doesn't just want to rely on green K, so he gets his armor upgraded, and is nearly as strong as Superman is, but Superman is slightly de-powered, but we can still see them fight, then maybe that fight would be more enduring.
Supes could watch out for Kryptonite blasts in flight.