Tow bar mounting is, imo, the best option. Quick and easy, little risk of damaging the car or the bikes, no strength required to mount the bikes, out of the wind, stable and lockable. The Thule one I have has a tilt action so I can still open the rear to get the dog in and out with the bikes loaded.
Also, the tow bar ones usually have electricals, so you don't need to worry about blocking the plate and lights with your bikes.
I guess an estate with bikes inside will be fine if you only have front passengers, but most estates would struggle with 4 people on board, 4 bikes in the boot, and luggage.You've bought an estate and you want to hang the bikes off the outside?
You've bought an estate and you want to hang the bikes off the outside?