I'm libertarian in the sense that workers should be free to consolidate and be as powerful as those employing them. Modern mainstream libertarianism fetishising private business and belief of the free market handling everything is ick.
But then again, I'm a noob to all the schools of thought so Benji will probably pwn me.
Libertarians are a distrustful and cynical sort, but the traditional "left" to "center" libertarian wing (with reason as arguably the most "centrist" libertarian publication) only opposes state advantages granted to unions especially when it creates a monopoly or monosopy in employment, this tends to lead to reactive and negative feelings against unions in general, but if you pin them down (they're wimps, which is why they don't oppose gays and darkies like real libertarians) they'll concede unions in the private sector.
Now, in the United States, this often forms into a debate of how much the state is putting its finger on one side of the scale or not in favor of unions. Much like debates over energy subsidies descend into debates over who is getting the most and how.
It's the subsidies/protections/etc. from the state that are seen as going against the free market, not employees voluntarily pooling their labor to negotiate or even strike.
Though, the standard libertarian view is far more one of "then employers can just fire em and hire non-strikers" than it is willing to factor in the power relations dynamic beyond the legal ones. I, personally, vary depending on the industry and in some I just think pursuing organized labor is more silly than anything like frickin video games which GAF saw as the solution to the industry's cycles. The unbalanced power dynamic there may be being exploited by the employers often but one reason they have it is because well, look at the people who left Infinity Ward to form Respawn and what EA offered them, or Activision's treatment of Bungie.
But my view on any of this is probably colored because every union I've been forced to, or happened to, join has been dogshit and run by the worst type of people. They did little more than funnel dues to the top, then shit on the workers they were negotiating for. And the one I'm pretty sure they manipulated their elections because there were African dictator style victories happening. (Years later, after I had long since left, they used another larger union to help them push out that union through a classic union war that included good ol tire slashing.)
Though this is still my favorite worst union I've found out about but mainly because of the smug lawyer regulator dude:
