Trouble is that some of the residents will take it too far, abuse their powers, etc.
I was in the police in the 80s when neighbourhood watch was a big thing and our group were tasked with liaising with several groups which meant us attending local meetings in church/village halls etc to help set them up, patrolling the areas whilst meetings were taking place (as it was thought burglars may target areas on meeting nights if they knew lots of residents weren't in their homes!), random visits to the areas when out on patrol duties, etc. Most ran pretty well, and those involved followed the guidance, i.e. don't approach dodgy people, note number plates of suspicious vehicles, generally keep an eye open for suspicious things, keep windows and doors locked, valuables out of sight, etc., and they brought communities together as people started looking out for each other when their neighbours were out etc.
Unfortunately in some, there was the over-officious types who took over and actually started "patrolling" their streets, generally making a nuisance of themselves, knocking on neighbours doors "telling" them to close an open window or to remove visible valuable items from their car etc., and some even making citizen's arrests of people doing things that even the regular police wouldn't arrest for.
I do think it needs to be restricted to proper/respected security firms, the kind who already exist to patrol/supervise commercial premises, events, etc., so that there is some kind of basic training, supervision and management of them.