As someone that worked for a non-union police department, and has worked for unionized departments, the answer is somewhere in the middle. In non-union departments it becomes a training department for others: they hire someone for lower wages (because they can't collectively negotiate better wages) and then that person leaves for somewhere else, taking the investment and experience with them. In union departments, they play politics and waste money and time on lobbying.

There should be unions, but of a semi-limited nature. They have collective bargaining rights, which they use to take care of their people, ensure fair wages in the area, and remain a competitor in hiring qualified officers. They should be restricted from lobbying on legislation (unless asked for comment, in which case an official report should be generated), and restricted from making political endorsements/contributions as an entity (privately is fine).