Okie, I've got time to actually respond to this now.
The question cruise asked was what are the disadvantages of unions. Not are unions the second coming of baby jesus or are they evil. So, from my limited (one year) perspective, here you go. Keep in mind, for every negative I am listing here there are probably five positives.
While unions do protect you from unfair firing practices, they also protect some people who shouldn't be protected. There are about 3 captains at my airline that would have been out on the street a LONG time ago for doing really stupid stuff except for the fact that the union has protected them every time. Now, there are also probably 10s of people who have been accused of stuff by the company unfairly and would be on the street if it weren't for the union, but the side of effect is that there are several captains that are in fact unsafe currently flying the line. Pro Stands (the union really) can only talk to them so much and they know how to play the game so nothing ever really happens.
A union is made up of it's member's and technically a union's leadership is supposed to react to the will of the members. However, it is possible to have leadership that becomes out of touch with the members and does things with out their consent. We now have a clause in our contract that states that ANY decisions regarding quality of life and compensation must be voted on by the pilot group and not just the MEC. Here we have company protecting the members from the union leadership. Our current contracted was voted down by something like 85% of the membership. However, our leadership signed on the dotted line anyways saying it was in our best interest. They did this on two other occasions before the protection clause was put into the contract.
And finally, ALPA specific, it is very difficult for a national union to represent a major and their feed without running into a conflict of interest. I'm not even talking about the whole who gets the new airplanes argument. In a general sense, growth is always good so any union should be lobbying for new aircraft on property. Obviously, new 90 seat RJs mean no new 737s and likewise from the mainline point of view. What I am referring to is the fact that a mainline union can reinterpret a LOA or a deal made at their connivance and force it on a feeder union because they are both ALPA, and regrettably 2% of a mainline pilot's paycheck is worth a lot more then 2% of a regional pilot's.
So, are their disadvantages? Certainly. Are there MUCH bigger advantages to being unionized? Hell yes. But to simply say, no, there aren't and don't talk about it because it was much worse before doesn't really solve anything.