I've used all three external racks at various points, over the years
Granted, I've not used any of them for about 15 years, so fittings may have got better
Whilst a 'strap-on' rack is easy (if the cars rear allows fitment), they're not the most secure & can slacken off, and sway about
Roof-racks can be a stretch, particularly if you're a short-ass (I'm only about 5'9"), or it's a tall car..... MPV, etc...
We had a (manky, nasty) Ford C-Max, before getting the (sensible, logical, economical) Octavia estate, & that was a pain getting the bike (when it went up there) onto it, as was filling a roofbox
Plus, there's the weight of the bike to consider, that you're trying to lift to shoulder/head height
What may be forgotten about, if it's a MTB/CX bike that's going on the roof is the soil/grit/mud that will fall off it
Some soils may be more acidic than others, &
attack the paint, or the same detritus will blow in slipstream/wash there if it's wet, into the rear hatch surroundings & can damage the rubber seal
Yes, you need a tow-bar, & 12N electrics (or maybe a 13-pin socket nowadays), but you can also use a trailer if you need to (cheaper than hiring a skip!!)
- bike's out of the way (fuel economy/'wind noise')
- car-park height barriers aren't a problem
- bike's are visible, at all times, through rear-view mirrors
- modern light-units (LEDs) may be offered, by rack manufacturers (ie; Pendle) or can be added, as can forward facing marker-lights (if you want them??)
I had another couple of racks after this, but don't believe I have photos, this dates back quite a way, as that's a 1989 Marin Pine Mountain (which was px'd in late 1990/early 1991 for an RC100!)
The other factor is what type of ball your car has
A 'Swan-Neck' will require a totally different fitting to a two-bolt (if a 'clamp-on', to the ball, it may need degreasing, to stop movement)
Mine were all two-bolters, one even had a wide enough mounting plate to act as a mount for the 12N & 12S sockets
I even had a Bedford Rascal at one point, that acted as a bike-carrier, and a mobile changing-room
Scary as hell, on the m-way though, when a National Express coach overtook, or on a diesel covered roundabout
NB; even a 'strap-on' rack may obscure the lights, & registration plate, so may need a lighting board
Edit @ 17:15
If it really boils down to it, here. the car we bought for daughter has fittings for a 'built-in' carrier, if I remember the TV adverts correctly
However it does appear that it was a factory fit option
http://www.autoexpress.co.uk/vauxhall/adam/59804/vauxhall-adam-gets-flexfix-bike-carrier