Also, you learn hearing the difference in sound a car makes as it moves over to give room.
In short, I’d wager that < 0.5% of all people that have done any sort of serious biking would prefer to ride against traffic.
Yeah relative speed goes hand-in-hand with head-on collision and less reaction time and expectation that I mentioned. I do bike a fair amount, but not on major roads. I am coming to realize that going with traffic is safer, despite it being hard to see cars coming from behind you. I also didn't realize that it's illegal in most places to bike against traffic https://roadbikebasics.com/ride-with-or-against-traffic/
Bicycle mirrors aren't comparable to automobile mirrors: Car mirrors are stable in alignment - your body is in a relatively fixed position in the driver seat and so are the mirrors relative to you and the car. On a bicycle, your handlebars are moving frequently as is your head. The whole bicycle vibrates far more than a car, sometimes making the mirrors unusable. Cars have space for more and bigger mirrors.
(Also, isn't it risky attaching anything to your helmet? In an accident, as far as I know, it can both change the helmet's performance properties and also get jammed into your head.)
A lot of people consider it a cute gadget before they tried it and regard as essential safety tool once they did. It certainly depends on your use case - in busy cities it's useless since you'll be alerted to cars constantly. However in my area I ride on countless long stretches of road with little to moderate traffic. Cars will surprise you on these kind of roads. Sometimes you don't hear or notice them.
With the Varia radar I'm much more aware of my surroundings. I can take up the middle of the road until a car approaches. I won't be surprised by cars I fail to hear due to strong winds or other noise. I know when it's safe to slalom around obstacles on the street. Or when to do a left turn (I'd still look but I'm not sure if the radar ever missed a car - it's very reliable). These days I'd feel almost naked without my radar.
Plus, don't forget that it's also a very good rear light which you can control from your head unit. Don't underestimate the importance of daytime lights in order to be noticed by cars - that is the single most important measure to avoid accidents on a bike.