I prefer roof mounted racks for week in week out convenience. So convenient to ride upto the car at the end of a ride and dump your bikes in the racks in a matter of seconds. The mud on the roof doesn’t bother me – it washes off pretty quick when the rain next comes round. Also it amazes me the number of people that are concerned about getting a bit of mud on the roof of their car but are happy to put a muddy bike inside the car.
I use the Tow bar mounted rack when going anywhere with the family – with an SMax getting 4 bikes on the roof is a pain for someone of my height getting the two bikes on the inner positions. but the tow bar rack is less convenient – you have to be a bit more careful loading the bikes to avoid contact, parking is a problem due to the significant extra length if you have a 4 bike carrier, so not feasible to leave on all the time. And it obscures access to the tailgate even though it tilts out of the way – it’s good to be able to sit on the back of the car when you’re getting changed and having complete access to the boot. I also use the tow bar rack when going to France on hols – I remove the roof bars for max mpg, and no need to use the rack when over in France, so the rack comes off and goes in the boot.
I used to use boot racks back in the day – frightening. Never had a problem with them but they do wobble about a bit, render your tailgate/boot useless and are just a general faff, and all that contact with the car body makes me wince.
So none are the ideal solution for every situation. Get a van for that!