Libertarians (of which I am not) are not against unions, they are simply pro freedom. Freedom means being able to assemble, but it also means not being compelled to assemble, especially when a portion of the dues are used to fund political action for policies they don't support.
EDIT: autocorrect is horrendous
Freedom also means you can strike, walk-out, or quit your job if you want. Union shops exist because unions have the power to have everyone else walk out if someone is hired who isn't in the union. That's how the concept was originally created. (I think the formalism around modern elections, created by Taft-Hartley, is actually strongly anti-union but that's another matter) it therefore seems odd to argue that you should be free to work in a union shop without joining the union, with all of your union co-workers forced to not walk out in response.
Or just speaking practically- a good union is democratic, and the democracy is much smaller - you can have more impact if you speak up- than the big democracy of the federal or state governments. So shouldn't taxes be a larger gripe than union dues? A portion of my taxes goes to fund all sorts of things I don't support.
I think the free rider problem you allude to is a result of union membership providing diminishing value.
But again, the only part I really have an issue with are public sector.
PS. You're not wrong about taxes, but let's not use debating fallacies please. We're talking about unions.
No, free riders are a problem regardless of membership value. You say that you are for unions but you are against the only thing that allows unions to exist.
I support the individual. Pretty simple.