It’s fear mongering. Sanctuary cities reduce crime by letting undocumented immigrants report a crime Without fear of being raided by ICE. The more people reporting the crime safely, the more likely the perpetrators will be arrested. Since they are often repeat offenders, taking them off the street helps reduce the crimes.
If the victim were to scared to report however the criminals continue. Why opponents of sanctuary cities don’t think about that is beyond me. You think they’d want to stop felonies but they focus on misdemeanors.