That said, I have been thinking, why ? Essentially it is running. Nobody I know is really that interested in cadence when running. I sometimes look at it afterwards but it just doesn’t seem to vary much not like cycling where you have gears. What cycle specific sensors are you going to have ? Power is out. You can get speed from gps. Heart rate is common to both.
I think it's mostly about handlebar-friendly form factor, and probably navigation. You can't look at your wrist easily on a racerunner. (It occurs to me at mounting a watch to handlebars isn't a particularly difficult problem, but presumably you lose the heart rate feature.)
Friend also has an ICE trike, so a cycling-specific unit would make sense there, too.