They're a mixed bag, honestly. I notice the types of people suspicious of teachers unions feel more comfortable with police unions, and vice versa. In both cases, the suspicious tend to think unions protect the corrupt and incompetent.
In my 20s I worked for a well-known union shop, was a member of the union myself, and I encountered example after example of laziness and incompetence that couldn't be effectively held accountable. Promotion was mostly on the basis of seniority rather than merit. It seemed obviously inefficient and wasteful.
Whether or not your suspicion mostly lands on police or teacher unions, you will assume that in it the work environment will be similar to what I experienced in a union environment. I suspect you'd be right about that whether it is teachers or police who have your suspicion.