My experience is that some trunk racks take too long (and require too much care) to set up properly, they can scratch the car (where the mount touches the car, where the straps attach, and where the bike pedals might swing into the car), and I've always had anxiety over its potential failure (I feel like a failure can damage both car and bike, whereas with a hitch rack the damage to the car at least will be minimal if say the rack somehow fell out of the hitch).