ACAB is a common slogan, especially in anarchist spaces. Should we really be using it though? It is a reference to children born without their parents being married, and due to christian morality is seen as inherently negative. It is effectively a slur. Do we really think that trying to enforce the hierarchies we are trying to get away from on others is going to help us? How have we allowed this slogan to become so common?
As an anarchist I think we should be defending these people, not punishing them with the hope of some of that transferring to cops.
Police unions are not unions in the same way a workers union is a union. They exist to uphold state sanctioned violence and protect the perpetrators of said violence, not create a strong working class through solidarity.