Beside different mounts, the mirror shape, size and convexity are also to be considered. Strongly convex ones might be good in city traffic as they give a wide field of view, but for seeing distant (but fast) cars on country roads a flat mirror is better.
In practical terms, as everybody's bike, body, riding style and road environment is different, it can be very difficult to find a mirror or mount that works well for you. I tried literally dozens. I recommend, before buying a good expensive one, buy a cheap one in a 1-dollar/1-pound/1-euro shop and tape it to the handle bar in different positions to get a feel how much you can see and where the best mount point would be for you.
Besides bicycle mirrors, it's also worth looking at motorbike, e-bike, mobility scooter and wheelchair mirrors. I'm now using a motorbike bar-end mirror (aluminium frame with real glass mirror), which has the advantage that it's more solid than any bicycle mirror that I could find and doesn't wobble or fall off at potholes (of which there are plenty here), and also has better optical qualities.
