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.
The adversarial situation you want to avoid already exists: Your boss wants to pay you as little as possible; you want to get paid as much as possible. All you're doing by disdaining collective bargaining is sacrificing leverage.
Our owners are not as naive as we are: https://www.mercurynews.com/2019/05/24/when-rules-dont-apply...
I find it frustrating to discuss the failure of unions without being labeled anti-labor. But it seems to me, the fundamental failure of unions, even when they seem to work, is setting up an adversarial system. I would posit for any social institution to be stable you need an alignment of incentives.