I have no concerns with private sector unions. With a private business, management and the union have the ability to negotiate and enter into an agreement which should be acceptable for both sides. Most of the complaints I see regarding private sector union contracts relate to management making poor decisions. From my perspective, caveat emptor. If you get taken advantage of, don't come running for support.
Public sector unions are something else entirely. The politicians who are supposedly negotiating on behalf of the public are receiving campaign contributions at the same time from the union who sits across the other side of the table. That is a ridiculous conflict of interest. In my mind, public sector unions should be required to give up its ability to engage in political activity to keep its right to collectively bargain. Under the current rules, the unions essentially are paying the other side of the negotiating table.