It depends on the mirror
A lot depends on the mirror.
I use either the Mirrycle:
or the Cyclestar:
I started using a mirror on a recumbent and missed one as I got used to it.
Both of these are solid and stable enough to be useful tools. The preference is the Mirrycle, but for folding bikes and racing bikes, the smaller size of the Cyclestar and the different arm lengths makes them more adaptable.
Mirrors are NOT a replacement for looking over your shoulder prior to a manoeuvre or approaching a hazard, but do have an important contribution in general riding to increase your awareness of what is behind you.