michelda is correct, with less than 200m RVR you need an automatic rollout control or guidance system. However there are still distinctions between different systems and DHs, although all are considered CAT IIIb.
For example the 737 has only a fail passive automatic rollout system if one engine has failed above alert height (200ft). Therefore you need a DH now (50ft) and the minimum raises from 75m to 125m. Same if the autoland system reverts to LAND 2 status.
Autoland on contaminated runways is allowed, however boeing cautions that autorollout capability might not be enough to cope with correction demands. Pilots therefore have to prepared to disconnect the autoflight system and take over manually.
AFDS ROLLOUT mode performance cannot be assured when used on contaminated runways. The ROLLOUT mode relies on a combination of aerodynamic rudder control, nose wheel steering and main gear tracking to maintain the runway centerline using localizer signals for guidance. On a contaminated runway, nose wheel steering and main gear tracking effectiveness, and therefore airplane directional control capability, is reduced. To determine the maximum crosswind, use the most restrictive of the autoland crosswind limitation, or during low visibility approaches, the maximum crosswind authorized by the controlling regulatory agency.
CAUTION: If an autoland is accomplished on a contaminated runway, the pilot must be prepared to disengage the autopilot and take over manually should ROLLOUT directional control become inadequate.
Personally i would only use it if there is no alternative left, as the above is a typical ass covering exercise and therefore if something does not work out as planned it all lands squarely at yours truly.