I used to haul two bikes to and from college in Santa Barbara from the Bay Area on top of my old 325e with a Thule fork mount rack on it, plus the front wheel holders. I could shake the car by grabbing the bikes forks and moving it. Doesn't get much sturdier than that IMO. My boss has the Thule rack that clamps down on the front wheel, and we haven't had any issues with that either. Just be aware that bikes on top = bugs if you decided to go that route without a cover.

Now I have an old Jeep with a VERY hold hitch mount bike rack that was behind the garage at my Grandparents house that they had forgotten about. Its tacky, kinda loose (shimmed with carboard its a lot better), but assuming I'm not traveling very far, I have no issues with it at all. Before the hitch mount rack materialized, I fit everything in the back seat, which worked, but left zero room for cargo.

Before ALL of that, I used an old, and I mean old, Hollywood trunk rack with just those few metal brackets holding on, and never had any problems there.

I'm not afraid of racks, either roof or rear mounted, and I have had zero problems. If you get rear ended... well sadly there isn't too much you can do there. Apparently I'm not prone to having that happen. If you forget how tall your rig becomes and drive into a parking garage (like a coworker with his Tarmac on his MINI... Tarmac = dead, MINI roof = dead...) then you will have problems. I personally never have.

YMMV