I have nothing against private sector unions.
I do, however, oppose public sector unions.
The reasons?
1.) Virtual monopoly
The nature of public sector employment, policing, firefighting, teaching, public works, etc, grants public workers a de facto monopoly over the particular service they provide.
If the police union, for instance, goes on strike all policing within a given jurisdiction can, theoretically, come to a complete halt.
Nobody should have that kind of collective power to cripple the voters.
2.) Collective bargaining
Setting wages for a particular government job should be the responsibility, and within the power, of the people that "we the people" have elected to do that job. "We the people" have not, so far as I'm aware, ever voted to allow union officials the power to collectively bargain for our public servants' wages. The same goes for collectively bargaining for public employees' work responsibilities or limits on what public employees are asked to do.
3.) Collusion
Given the above mentioned (unauthorized) power that unions wield, they can and do trade that power for votes. Politicians promise to increase wages and benefits and unions in turn promise to throw votes toward a particular party.
If union wage demands drive up the cost of a Ford Explorer I always have the option of buying from a Ford competitor.
But when it comes to government there is no competitor.
Public sector unions should be eliminated immediately and I will side with the government and the people in breaking the back of public sector unions whenever and wherever I see it happening.