This is a tricky question. On the one hand, the vibration administered by shock collars is, technically, safe and has not been shown to cause damage. Used liberally among hunters, they do work well when training pups for the hunt.
However, there’s an argument to be made that shock collars are inherently inhumane because they can cause fear, anxiety, and aggression in some dogs.
Moreover, some malfunctioning shock collars have caused electric burns.
Generally speaking, shock collars are an extreme tool that should only be used sparingly by trained professionals in highly controlled environments.