Remember, the rear of the vehicle is not built as strong as the front of the vehicle and does not have the engine to help absorb energy in a crash. Also, someone sitting in a rear facing seat, in a rear end collision, will have their legs utilized as crumple zones. And in a rear collision the head, neck swing toward the rear of the vehicle (main reason for headrests back in the 70s). Facing the rear means the neck will snap forward. And I believe, rear end collisions are number 1 (?).
If the vehicle was designed for these seats, usually no problem (I believe Mercedes has a model with rear facing seats, but the thing is built like a battleship), but if not, it was due to crash problems as well as money problems.
If you are the person hitting the car in front, then this third row seat might be the safest place in the vehicle, so long as they come with head restraints.
Just a word of caution; makes no difference to me personally.