For what it's worth, I routinely carry very expensive, very heavy, DH race bikes on my car.
I very much like my roof mounted carriers. I've never had an issue with them and they support my bikes very well while still giving me easy access to my hatch. But they are noisy (even without a bike) and they seriously hamper fuel economy (without bikes about 1L/100km lost, with three bikes up there, about 4L/100km lost).
If I was doing it again, I would get a hitch mounted carrier (with a $20 locking hitch pin from the auto-parts store of your choice).
Get a wheel-tray type that supports the bike from below, not one you hang the bike from. There are lots of different bike shapes out there, and they don't all fit on the hanging style. Don't skimp on the carrier, a bad carrier can damage the car (by letting the bikes swing into the trunk) or drop the bikes (happened to a friend of mine at 40mph). Thankfully a good hitch carrier costs $250. Yes that is $150 more than the cheapest, but if the bike carrier ever fails, it'll cause more than $150 worth of damage, so you might as well spend the money on a well made one that will last forever.