Well even though I use the roof mount most of the time, I feel like the hitchmount needs to be defended
Once you live with it for a while, the convienence and speed of use is soooo welcome! Just plop your bike on there and lock. Thats it!
Now there are rear end collision concerns but there are a few options:
1. I noticed that Thule has a hitchrack that sets the bikes higher so that even if you were rear ended, it would not hit the bikes. Maybe it's so you can also tow as well as using the hitchmount.

2.If you have a taller vehicle, like a truck or suv, the bikes are sometimes high enough that at least a car rear ending you is not a issue for your bikes.

3. The actual hitch is really strong. Mine is rated to 5000 lbs towing. Someone rear ended me and really smashed the front bumper. The tow hitch stopped him. I didn't even get a scratch on my truck. Obviously, if my rack was on there it would have been toast but a rack and bikes are pretty unusual and highly visible.

4. I'm not sure how a fork mount gives any more stress than jamming on the bars during a sprint or out of the seat climbing, throwing your bike around, etc.
I'm not saying upright mounts are bad. I just think they are less stable than fork mounts, harder to use and take up more room (at least with Yakima's uprights, the actual racks are wider than fork mounts so 4 bikes isn't an option.

5. Hitch can be rear ended but the bigger and much more common accident is when people forget about their bikes on the roof and hit the garage, low sign, drive thru roof, etc. so accidents to your bikes are actually more common with a roof mount.

anyway, like I said, both roof and hitch have their pluses and minuses.