While the demographics of CR subscribers are clearly different than the full population of owners, I don't see how the demographics differences between the two groups would affect the outcome.

The individuals merely report issues with the vehicle. Their results may vary as compared to the general population, but, the relative percentages between vehicles will probably be similar.

I value the statistics as provided by CR. What I question is their subjective evaluations of the vehicles. Here is where the bias of the driver will definitely come to bear. The driver should be escorted into the vehicle without being able to see it's moniker. All interior badging should be removed. Let the driver evaluate the vehicle without knowing, in advance, who the manufacturer is. Bet the results might be significantly different.