I'm going back a few years so I may be wrong, but ISTR that one of the problems witha mu-meter was that it gave inaccurate readings if the runway was wet or contaminated.
A snippet:
On wet runways friction measurements are carried out in order to establish whether the runway is slippery when wet, and to indicate the need for runway maintenance. Friction measurement on wet runways is NOT carried out for operational use by aeroplanes, as no correlation exist between measured friction and aeroplane behaviour.