The fundamental problem with TNRM here in Hawaii is that it guarantees that the colonies will continue to increase in population with no reasonable end point. Any non-neutered/non-managed cats will add to the population that eventually work their way to a nice colony area with guaranteed food.  If you've seen colonies of cats, you recognize that there are "outsiders" (typically aggressive males) that harass the nearby cats and that avoid humane traps.  When these colonies are near other houses with domestic cats, the outsider males begin to also interfere with the house cats, causing behavior problems for those households (marking furniture, peeing on everything).  TNRM sounds good as a short term concept, but when put into practice on an island with no winter and no other predators, it is simply a recipe for a slow progression toward infinite cats.