I believe that as far as the UK is concerned mirrors are now a no-no for new installations for the CAA...... and old installations should have a plan for replacement/phasing out.
Mirrors require an agnis to keep on the line (OK .. mainly for night hours as everyone can see the line in daylight
)
CAA don't want systems where pilots have to look at two different places for guidence.
(So agnis + papa are also on the way out)
Mirrors can work fine unless there are multiple stop bars, the mirror gets knocked out of aligment (no vision) or the electrical heaters in them stop working so they mist up/ice over.
edited to add: of course mirrors don't work for the larger a/c types ....