I used one of the top spec Thule 4 bike tow ball carriers on my last car and it was very good. Very well built and gave me confidence the bikes were safe.
Since getting a T6 I have used the VW tailgate 4 bike carrier and I prefer it, absolutely rock solid. Tow ball carriers are good but with a single attachment point they will inevitably move a bit especially with 50kg of bikes going down bumpy country lanes whereas the tailgate carrier stops them bouncing around at all.
In non covid times I use mine 2-3 times a week so it’s great to be able to flip it down and load rather than having to lug a carrier out of the garage first, but if you’re only using occasionally this probably isn’t a consideration.
The tailgate carrier does mean you’ve got to lift bikes quite high for loading, I’m 6ft and keep a small collapsible step in the van for getting the bikes on and off. With bikes in place It does raise your overall height to 2.3m with average adult bikes in place, when not in use it is very slightly lower than the main roof height, they’ve designed it very well.