As mouse-killers their efficiency varies enormously. Few of them really come up to scratch as they cannot often get into areas where mice can live in a building.
Feral Cats (Felis catus) colonies can carry a range of diseases that place people at risk, particularly if this occurs in hospitals, food production sites and other high risk areas. In addition they cause significant unpleasant fouling, smell and contamination. Feral Cats can also be aggressive and interfere with the management of areas where ducting and other habitats that they occupy need to be visited by electricians, plumbers, cleaners and other trades.