I put a bar mounted mirror on one of my road bikes and never used it- I had to keep adjusting the tightness of the fixing screw as it kept moving. about a year later I started using one of my other bikes and found I was looking for the mirror that I "NEVER" use.

The bar mounted ones are easier to use than the helmet version but you can adjust to the helmet one and I never managed to stop the bar one from moving. No longer use one but I have good ears- a Flexible neck and mainly ride on Backroads with little traffic.