Dick and Fuji - WAM in Australia is approved for 5 NM separation in Tasmania and 3NM separation in the air in the Sydney basin where it also provides Precision Runway Monitoring (PRM). On the ground at four airports it is used for surveillance and provides situation awareness, not separation. The accuracy of multilateration is not in doubt but the number of ground units has to rise to meet the accuracy requirement. For example Sydney airport has approx 14 ground stations for surface movement; Tasmania has approximately the same number to cover the whole island.
Incidentally each multilateration receiver is also an ADS-B receiver and only needs one station to pick up a position. Multilateration requires between 3-4 stations to be receiving a transponder to establish and maintain an accurate track.
Mutilat which requires a transponder (preferably Mode S) is a great surveillance tool midway between SSR, which also requires a transponder, and ADS-B, which requires a new piece of equipment. ADS-B is the way of the future until someone invents something better.