Friction tester are fairly accurate, when used on contamination they are approved for use on! Torp uses the Skidometer/SKH, and it's only approved for slush 3mm or less deep. (National regulations, rules for all the other friction testers and contaminations as well)
We have had several cases in Norway, when friction testers were used on non-approved contaminations/depths. The Griptesters/GRT used on the STOL airports is over-represented in these cases (Because the GRT is a pice of s***)
Tapleymeter is not used anymore, other then as backup when primary (SKH/SFH) fails, and the restrictions on use are tight.
The SKH used by all civilian airport with long runways (Except OSL, they use SFH) have it's problems, but mostly in the "safe" direction with dry snow contamination. The conditions reported are often worse then experienced by the aircraft.
The Saab Friction tester/SFH used on all military airfields and at Oslo Gardermoen is the most accurate means of measuring frictions, but restrictions on use are the same as the SKH.
A data link system in use by Avinor at Oslo and Tromsų, and by the Air Force at Bardufoss has an in-built error blocking feature. The operator cannot input illegal combinations into the condition report. If he tries to input friction data from the SFH , when he has input let's say 6mm of slush, the system alerts him to the fact that he has measured friction on an non-approved contamination. We have seen much improvement in condition reporting due to this system.
However, the main factor i many of the above mentioned cases, was old friction data. (The Premiar A330 at OSL case in point) It does not matter if the friction test was 100% accurate if it's 5 hrs old, and it has been snowing since then! Accurate reporting is much more dependent on the personell in charge, then on the equipment. At Torp the friction test was 17 minutes old when the airbus landed. If it was snowing on a slush covered runway, that is a lifetime!