I've transported a 40 to 50 pound tandem a dozen or two times on a Saris Bones 3 and would gladly do so again. I do take the wheels off and strap the handlebars, and the bike may hang a bit off each side of the car, so a bit of care needs to be taken.
I wouldn't do it with 4 strap hanging racks, but the 6 straps on the Bones seems to be just fine. I do *really* tighten down the straps, and then after everything is tight I push down a bit on the bike and tighten the lower straps a bit more as they like to loosen when you load the bikes.
For those who haven't used one, the Bones is substantially more secure than any of the similar looking old style truck mount racks I've used.
The only annoyance of trunk vs. hitch racks is that I feel the need to remove and throw them in the vehicle while I'm out riding as they can be easily stolen from the car. On longer trips, even with a lock running through the bikes to the frame of the car, I will always keep the bikes in sight during a stop, but I do that with any rack.