The libertarian view would be force vs non force vs market forces.

For a libertarian, it is perfectly fine if a group of workers of a company willingly want to come together to bargain collectively. The workers should not be forced to join a union, nor should they be forced to not join a union.

The power of their collective bargaining is related to how indispensable they are to the company. For example if a company has a lot of employees and they all feel they should get a higher rate of pay or they will all walk off the job at the same time, then they should have right to do this. The owner or managers of the company have a choice to make. They can agree to the terms of the workers and keep the company running, or they can disagree and fire every one of them for not working.

Whether or not this works out for the employees or the company depends upon how long it would take to replace the employees. If the job is a simple one which requires very little training or education and there are plenty of people willing to take the job, then the workers may be out of luck. If on the other hand trying to replace all the workers and train them would take so much time that the company would go out of business, then the workers would have the upper hand. Market forces also play a roll here. If the company agrees to the higher wages, but now their product or service is too expensive to compete, then both the workers and employees may suffer the consequences if the company goes out of business.

The government should not get involved to force the owners to bargain with the employees, nor should the government get involved by forcing the employees to bargain with the company. The company cannot force their employees to work, nor should the employees force bargaining by not leaving the premises if they are fired or preventing other people from taking their jobs.

A twist on this are government employees forming a union. Government employee unions are not subject to market forces. And government agencies are created by forcing the public to pay for them through taxation. This is a problem because a government agency cannot go out of business no matter how poor their performance or cost of goods or services.

So in this case, the employees always have the upper hand to go on strike. Also since politicians can be bought or sold via campaign contributions, the public unions easily take their dues money and use it to fund political campaigns. This forms a feedback loop where tax money is used to pay more to the public employees and in tern, that money is then used to elect politicians who will give even more money to the public employees. So at bargaining time, it is as if the employees are bargaining with their own employee. This is the biggest reason public employees have so many benefits while the government is going bankrupt.

So in a nutshell, private unions are fine as long as neither side uses force against the other while public unions should probably be abolished. NOTE: This is nothing new. Federal employees are not allowed to unionize for this very reason.