The main feature is that the wheels locate in a moulded profile, which holds the bicycle in an upright position. This means that you can use both hands when securing the wheels and the frame. When the bicycle is attached to the frame holder, the bicycle holder is secured to the load carrier.
We bought one of these about 8 years ago, as we did not have a tow-bar, and thought a back carrier would get in the way of the hatchback door. The clever part of the design is that the bike stands on its own in the channels, while it is fixed in place. The locks are interlinked, so that the bike is locked to the carrier, and the carrier is locked to the roofbars.
It always worked well,and with a new car and a second bike, we kept to the system and bought a new carrier. We had initial problems finding out how to fit the carriers to the new roofbars, which were the aero type, but the manufacturers were very helpful, sending detailed photographs, and bolts to adapt the old carrier.
The advantages are that the cycles are well out of the way, they are secure, and there is no problem with obscuring lights or number-plate.
The main disadvantage depends on your height and strength. It can be quite difficult to lift the bike on to the roof, particularly in a restricted space. It would also add to the price of most ferry crossings.