For normal landing performance, credit for reverse can be claimed (normally by the manufacturer) if reverse can be shown by certification to be sufficiently reliable.
AFAIK very few manufacturers have achieved this, probably because of the many interlocks to ensure that reverse will not come out in flight, thus increasing the probability of system unavailability or relatively ‘inconsistent’ response on the ground (details in FAR/CS25.125 ‘… means other than wheel brakes').
For contaminated runway operations, the European approach is that reverse thrust can be used, but the increased risk is mitigated by the assumption of low exposure – rare occurrences, as it is strongly recommended that contaminated runways are avoided, and also that the landing performance is calculated by standard certification methods.
Some US operators appear to have a FAA dispensation similar to the EU approach, but I am uncertain as how the risks are mitigated, particularly as contaminated operations are often taken to be normal. Thus a reverse ‘failure’ on a contaminated runway contains an element of increased risk, which if encountered on a limiting runway (considered to be always for a contaminated runway), then an overrun is most likely, particularly as there little or no safety margin in the distance calculations, as there has to be with non-contaminated operations.