I use both roof mounted and tow-bar mounted.
It's a Discovery, so roof-mount is a bit of a clamber, but OK for just one bike.
The tow-bar rack is big and heavy, and takes a fair bit of effort to haul it out of the garage and get it mounted.
And then, it blocks access to the tailgate.
Yes, it can flap down, but not easily when loaded, and not enough to gain access.
I will use the towbar mount rack for single big journey with 2+ bikes, eg going to a holiday destination, where I can then remove the rack and store it for the return journey.
Otherwise, if it's 2 bikes or fewer, it's too much faff and I'll just chuck them on the roof carriers which are always there.
I *can* fit more that 2 on the roof, but that involves actually climbing up onto the roof; the 2 outermost ones can be done by standing on the rear seats.
So it's just whatever works best for you.