I'm best used in small doses, but I am non-addictive and non-habit forming.
I really don't like boiling complex questions down to yes or no responses and I can't speak for unions or departments nationwide. I can only say that with how merit boards work locally if someone is legitimately unsuitable for police work and the department can't document that and figure out a way to solve the problem, the union probably isn't the major factor. My personal experience is the command staff is the major hurdle for someone who's simply unsuitable but not for politically sensitive reasons such as simple incompetence.