Nothing incompatible about libertarianism and unions. It's a free market construct, a free association of people formed to put a check on the power of wealth. With a long history of acting as a check on one of the forces that seek to destabilize a free market in favor of state control, one might add (corps love to team up with the state to gain power).

Though, caveat, libertarians might have a word against unions of state employees due to the mechanics of unions bartering against politicians who only stand to lose someone else's funds (the public), through state mechanisms which libertarians may oppose. Obviously this is more problematic with police unions than teacher's unions.

Of course, there's the FOX News "libertarians" who aren't thinking any of this through and are just rehashing whatever FOX News is slinging. Basically indistinguishable from year 2000 "conservatives" plus all the culture war and 4chan talking points of the last 20 years. (edit: They would most likely be anti-union across the board, of course).