Libertarianism is completely compatible with unions in general. You are free to join unions. If libertarians look like they don't support unions, I think it's because they become uncomfortable with the ways unions themselves sometimes (sometimes!) become coercive organizations themselves ("union thuggery" is a thing), we'd be more hesitant to encode into law that
certain unions have certain special statuses, and we also recognize the freedom of the employer to just fire the lot of you; I would say it's on the union to prevent that, not the government.
I'll concede as a pragmatic matter that unions may need a bit of extra protection under the law, but it's very easy to overshoot, especially once you've created this new special interest group (the union itself, as distinct from its members) that is going to lobby for ever-more protection.
A libertarian may not look "for" unions to a liberal, because we won't check off the list of things that they define as being "for unions" (mostly various legal protections), but that's all a point-of-view thing.
I'd also observe as a libertarian that well-intentioned "protections" that initially appear to be in the union's favor may not be; the aforementioned "pricing out" of the UAW's labor was in large part enabled by excessively-strong union protections. They would, ironically, almost certainly be better off today if they had less power yesterday. Balance is complicated. Excessively-strong legal protections also provide the environment where it becomes easier for the union to start acting on its own, without consulting its members or even occasionally in defiance of its members. We must not ignore that the union is a new entity with its own existence, we can not simply model it as "a group of employees". (If that worked, we could simply model the corporation itself as "a group of employees", the whole reason we have a need for unions in the first place is that corporate entities like unions or corporations can take on their own existence.)