Car Bike racks fall into three categories
(Plus tail gate mounted proper racks only available for flat backed vans)
The string and wire spindly racks that are often seen in the UK ties on over rear windows. I'm amazed they are still actually sold in the UK, should you be caught with on in most of the rest of Europe, you will be removing bikes and rack at the side of the motorway at best, and up before the beak at worst
Roof racks:
The advantage is they can go on most cars
The downside is fuel consumption, height, and the issue of lifting a bike to head hight
Tow ball mounted racks
Having used all four, the tow ball is the way to go, probably the most expensive but ...
It make no difference to your fuel consumption
The bikes are protected from 70mph wind and rain on all day motorway journeys, and even after hours of driving rain and motorway spray are still dry !
Available as 1, 2, 3 and 4 bike carriers
Legal all over Europe
Comes with lights and number plate, so no issues with blocked number plates or lights
Very easy to mount on the tow bar (30 seconds)
Rear boot/tailgate is fully open able (even on a van)
Secure
Easy to put the bikes on, at most you may need to turn the bars, but that depends in number of bikes and their sizes