I guess for a disk wheel it would not be as much of an issue, but the problem with wireless is quite clear once you have a group training session. My Polar goes NUTS when I ride together with a few of my racing friends. Doubt it's an issue with the current generation of power devices, but if the range starts reaching 3 feet (1 meter) so the receiver on your handlebars can receive the single from your rear hub, then it will also pick up signals from the riders left and right of me. Sure that is one reason most wireless speed sensors run off the front wheel.
Polar 520. Never an issue with the heart rate signal (range too limited). But have seen many instances of picking up signals from other people when I have the cadence enabled. Seems the cadence sensor generates higher RF than the speed or HR sensors.