That's why I would like to hear from someone who has tried one .I'd never heard of them either. I'm just guessing but I think that a user might spend a lot of time adjusting his wrist angle rather than concentrating on riding the bike or glancing over his shoulder..
I could be entirely wrong.
That may be but having an early alert of an approaching vehicle would be useful. I ride on mainly single track roads in the highlands and have only a limited width of road [often bumpy] to play with. Frequent head turning would soon have me in a ditch or i'd get a wobble on as some impatient tourist wants to squeeze by. Theres loads of ebikes and cars now and i cant hear them.Your ears and shoulder checks are your lifeline on a bike.