Police unions are much worse than other public unions. If other public unions take illegal action, the police can arrest them. When the police union does something illegal, no one is going to do anything about it.
as a normal rule the illegal actions I associate with unions are stuff like leadership skimming from the pension funds which would probably get them arrested.
the negative actions I associate with police unions is stepping in to protect corrupt or violent cops from disciplinary actions / firing / arrest.
They are allowed to work for pension increase, wages, vacation time, but not allowed to touch disciplinary situations?