Having driven under a low entry barrier with a rear carrier that lifted the bike higher than the car – it went under luckily. I will not be ever using a roof mounted carrier in any sort of hurry (bought an estate since then & gave that carrier away!).
Thule do towball mounted one (more expensive, if you don’t already have a towball) that will fold away from the rear door, even with the bikes installed, allowing access to the boot.
I have come across a guy (mtb holiday operator) who believed roof mounted carrier are bad for forks, as they stress the fork in a way there not designed to withstand. Even suggesting he’d come across people whose forks had some apart and the bike then flipped over backwards.
How true this is, I couldn’t say.