I'd suspect that the higher failure rate for white-box (versus brand name) PCs can be attributed to how the components are installed and paired up with other components. The assembly line style methods used by Dell and HP, where they purchase components of the same type in bulk and have workers who specialize in performing specific tasks; my belief is that would probably result in fewer defects better overall hardware configurations.

The guys who set up shop in a strip mall, buying parts from eBay or the local BestBuy, and then assembling them on a fold up table; I doubt the quality is there.