Yeah, they can be, so its more vauge than i've made it out to be. The underlying problem is that employee-employer is individual-collective; so in some sense, there "has" to be a union option if this becomes too one sided.

But a cartel is never required, since businesses competing with each other is the goal of a free market.

I would agree that if unions become more powerful than businesses, so it ends up single-business vs "union-of-unions" you can get the same effects.

Hopefully the asymmetry is clear though. Cartels are never permissible in a free market, but unions often are because of the need for labour to compete "on the same terms" as business.

ie., the mere existence of a union isnt a sign the market is non-free; whereas the mere existence of a cartel is.