I'm anti monopoly. Very much so.
I'm okay with unions as long as they don't engage in monopolistic behavior - I haven't found one of those yet though. For one, if unions didn't forbid company X from who they can hire from, and were okay with company X hiring pilots from any of the 10 pilots unions concurrently, then that would often be okay. Right to work laws are a super important step towards this.
Even with that step, union-monopolies and inter-union-collusion would still sometimes be a problem just like it is with companies, and that must be stamped out.