The build quality isn't as good as the real HK's I've seen/own. They have a known issue of the front takedown pin being off which means you need to open up the hole on a real hk trigger housing(be be fair you'd need to clip and pin it for a hk semi auto anyway). Another issue is that the 3 lugs of them are known to have issues(most seems to be the paint is too thick).
Its also worth saying that they were licensed. It doesn't seem like they are anymore. The guns are missing some upgrades made over the years due to this. The zenith's don't have the f bolt group upgrades in them that the sp5k does.
I also wouldn't really call it a clone as it's made from a factory that was at one point licensed using tooling to what was then HK's spec. I'm not saying they are bad though, I think overall the zenith guns are very good for the money(and have one). I would't say they are as good as the real HK ones.
I'd also say they are far better than the POF imports I've seen which is also from a factory that is or was licensed by hk.