Stability is not a problem for major OEM's because they only sell their PCs with hardward that they have tested to have no problems, which is very straight forward. Even if VIA is buggy, they work around it. Therefore a buggy VIA chipset would not caues them to switch to nVidia.
It would be stupid of them not to worry about stability. Sooner or later, the customer will add something that the OEM's haven't tested and a troublesome chipset is going to come back at them with a vengeance. Imagine how many tech support calls you have to take, replacements you have to send out, loss of confidence among your customers, etc.. Those 'workarounds' you mentioned costs money. With razor thin margins rigth now among OEM's, you need all the help you can get. And a buggy chipset wouldn't help you at all. No chipset is perfect but there are chipsets that consistently have more problems.quirks/incompatbilities, than others.