They use customer feedback (they just sent a survey out a month or so ago.....even to non members), recalls, warranty repairs/replacement (where they could get that info. etc.), their own personal inspections where applicable, etc. Unlike Consumer Reports, I don't hardly think they get one of every model to test. I believe their personal inspections are done on the lots or at RV Shows, etc. Of course the surveys will provide a lot of personal experiences but overall, one gets a pretty good thumbnail sketch of each make, model and year. And yes, they provide info on older units also.
Pricey (just like Comsuer Reports Auto Pricing) but for somebody that doesn't know and doesn't have the experience, I felt like it was a good starting point. Of course, like you pointed out, every make and model will have a lemon somewhere along the way. We've loved our BMW cars however, we did sue BMW under the California Lemon Law on our 06 330i. I still think and speak highly of BMW even with that one negative experience.