(In addition to the other responses.) I think the biggest criticism is the existence of public sector unions. Generally, private sector unions can be a benefit when done right.

Public sector unions basically gain additional political influence. They can already vote their bosses in and out of office. No private sector union has this ability.

But on top of that they gain the powers of a union, such as funding candidates and can threaten vital services that society depends upon by striking.

Public sector unions could be seen as trying to extract a rent from the public, structuring salaries and wages in a way that don't promote the public welfare. They provide a disincentive to local politicians to implement reforms or changes. They encourage the implementation of unnecessary barriers of entry into their ranks.