I think any discussion of the good and bad side of unions needs to distinguish between public and private sector ones.

In the private sector, union negotiations are naturally constrained by the knowledge that if the company isn't profitable, everybody loses. I think there are cases in the private sector where unions make a lot of sense (e.g. mining towns where there's one dominant employer).

In the public sector that doesn't apply. The union's incentive is to demand all that it can. You're not going to put the government out of business.