There are all kinds of reasons as to why one figure breaks and another doesn't.
From what I've seen and read about online with various KG figures, the breakage is simply due to some inconsistencies in the paint apps and putting the figure together when it was in production.
For example. Heavy paint gobs in a joint can cause that joint to be frozen up upon delivery to the customer. The customer torks the joint, and breaks the joint.
Another example of bad parts. In an ankle joint, I've seen both online and in my own standard KG, flashing that was not removed before the joint was put together. That extra plastic in there can cause added stress on the joint that wasn't design to support extra stuffings so to speak. And the ankle can break. That is actually, again, from what I've seen and heard online, a common issue amongst many that were "broken".
If the paint apps are clean, and the parts are put together correctly, the figure is actually quite sturdy, moreso I would say than the 200X line. The plastic on the figure strikes me as more giving that the plastic on the 200X line.
So what should Mattel do? I'm sure what they will always do their best to do and that's ensure good quality assurance. They are aware of these issues. But it's important to us to know that when you mass produce a figure in these quantities, there are bound to be some inconsistencies here and there, and they may or may not result in broken figures. It's gonna happen.
I think Mattel HAS addressed this issue beautifully and correctly by giving out replacements for broken figures.
They are looking into the Q and A of their figures constantly. (The recent upgrades on the DCUC series four figures should show you that.) And they do kindly offer replacements for broken KGs. What more should they do? I think they're doing fabulously!
