Quality control for all of the main model railway companies seems to be in the hands of a third party manufacturer on the other side of the world. As mentioned before, if you are prepared to pay for a fully checked, fully tested model every time the price will rise accordingly, not to mention the time to delivery.
Some of the smaller, cottage industry, manufacturers take it upon themselves to check every model which drastically reduces the number of faulty units, but the likes of Hornby, Bachmann and others with thousands of units handled every month can not afford to do that. They probably have a similar failure rate between them but what makes the difference to the customer is how this faults are dealt with. Individual units are the responsability of the retailer, there again some test every model before it leaves the shop for which the customer tends to pay a premium, others rely on cheap prices to sell as many as possible in order to cover the cost of replacing a handfull of faulty units (return postage, handling etc: The unit itself is refunded by the manufacturer).
Frustration for the consumer comes when the manufacturer suffers multiple failures or faults on the same item, batch faults or design faults. Some manufacturers are very transparent about this while others say nothing and keep on distributing units despite a known fault and leaving it to the retailer to pick up the pieces. Does it really do that much damage to a company's image to hold their hands up and say "we have problem"?