I disagree wholeheartedly with this idea that adversaries create instability. Stability is the result of all forces acting on an object summing to zero, to abuse metaphor. You can't have that if all of the forces are pointing in the same direction. There is a reason Nash equilibrium is called "equilibrium"; it is stable.
Unions fail primarily because they are a very shoddy bandaid. When you have commodity labor, in which each worker is the same, a union has the capacity to plausibly represent them as a group. That cooperation gives the workers negotiating power. There is A) no reason to suspect that that negotiating power cannot or will not be abused, and B) the premise is usually false. Police are the obvious target here on both counts, as a police union is nothing more than a private army, and policing is far from commodity labor. It matters a lot who my police are. I think that teacher's unions are probably a better example of the problems though because people tend to think of the police as an outlier.
