Incidentally, like most any aftermarket automobile product, there are no federal safety standards that apply specifically to add-on vehicle seats. That means there is no regulation or independent testing of these products whatosever. Manufacturers can and do claim they test to the same criteria as the federal standards require of seats installed by vehicle manufacturers, of course. It's possible they are very safe or not so safe, it all depends how much you trust the aftermarket company. As you said, there is also the issue of installing such a seat where it was not intended by the vehicle manufacturer.
For the previous poster, another example is the rear-facing seats provided by Ford and Volvo in some of their wagons. These seats are safe for occupants within the height and weight ranges listed in the manual. They are not suitable to install any child restraints. Whether or not there is a safety concern is unknown, simply because the child restraints aren't designed or tested for installation on side or rear-facing vehicle seats.
For the previous poster, another example is the rear-facing seats provided by Ford and Volvo in some of their wagons. These seats are safe for occupants within the height and weight ranges listed in the manual. They are not suitable to install any child restraints. Whether or not there is a safety concern is unknown, simply because the child restraints aren't designed or tested for installation on side or rear-facing vehicle seats.