I have four night guard, one on each side of my coop and run. They are not advertised to repel anything like rats and snakes and I am not sure that they really are effective at all. I bought them and put them up just as an additional precaution. My feeling is that they really do not do much because I have seen evidence of predators (bobcats, raccoons, etc.) coming around my coop and run and they have not been deterred by the night guards. I have a five wire electric fence around the perimeter of the coop and run and that is the real deterrent.
Have had the night guards up for a bit over one year and one of them has stopped working.