Wow....lots of problems with trunk racks out there. I have a Saris Bones 3 trunk rack myself which I have used to haul 2 mountain bikes up to 100 miles on with no issues. I've also used it to haul my road bike about the same distance. I've been on the interstate cruising at 80mph with no issues.

The other posts are correct about potential damage issues with them though. I have to be careful about how I mount it to my trunk to make sure it is very secure. Also, I usually tie something through my crank arm and back wheel to keep them from spinning. The front wheels are tied to my down tube as well to keep the wheel from moving back and forth. The positioning of the arms on the rack is very adjustable but you would definitely want to make sure that your bikes would fit on the rack, and on your car with no issues as posted above. No visibility issues out the back for me

Bottom line is a roof rack is better, but also much more expensive. That extra $ is well worth it though vs. loosing your bike off of the back of the car or damage to your bike or car if it does not fit properly. I only use mine 3-4 times/yr max and it works for my needs though.