I think a common Libertarian view is that unions are unfairly and massively promoted by the State. (For example, the absurd laws against 'retaliation'.) There is no objection to unions that arise through purely voluntary associations. However, employers should be permitted to do the same thing, and they currently are not.
You may be right that this is a common view among libertarians. In my view the state has more commonly hobbled unions, for example via the Taft-Hartley act and right to work laws I already mentioned. Or, going back further in history, mobilizing police and military against striking miners and workers. Today in many less developed nations unions face similar state repression via laws and police, part of the reason that overseas manufacturing is so inexpensive.
I think that fair is fair: nothing is off-limits during negotiation except violence. Unions can try their utmost to take employers for everything via contracts, and employers can do the same to unions. The employer's opening bid may be "you get $1 per piece produced, and have to buy your clothes at the company store." The union's counter may be "we get $30 per hour, retirement at 50, and lifetime pensions amounting to 70% of average monthly wages at retirement." Both sides can make ridiculous demands, but will eventually converge (if not rapidly and neatly) onto mutually satisfactory terms. But no cheating! No sending the Pinkertons out to rough up the strikers or strikers out to rough up the scabs. The state doesn't get to interfere either on behalf of either party, or on behalf of interested third parties like stock holders or bank lenders.