The trade labor unions are prospectively groups of laborers who bind together in search of a common vision and mission for the welfare of its members. They agree as a group on what benefits and rights the workers can enjoy such as salary, vacation, and special services. The by-laws and stipulations of a union are negotiated with the organization leaders, and compromises are reached that cannot be replaced or amended unless another negotiation takes place. That way, the leadership group must abide by the negotiated agreements.
The benefits of this are the creation of benefits for the laborer within the negotiations reached with the organizational leaders. the establishment of rules and regulations, and the solidification of the employer/employee relations using a legal process.
The drawbacks of this are the potential over-blow of power from one group against another, passive retaliation as a result of negotiated agreements (for example, you obtain your benefits but your employer treats you like crap and you cannot prove it because it is a perceived behavior), or simply the abuse of any laborer in summoning the union for petty things that would indeed slow down and break apart any good employee/employer relationship
There is never a disadvantage in forming a union. In fact, it is encouraged in many working environments just so that the employer can have a written set of rights, wants, and needs from the employees that can be negotiated rightfully and fairly without having any doubt as to what is at stake when working there. In fact, allowing a union might save an employer the headache of "misread contracts" or "perceived ideas" that employees might come up with once they become employed. Anything contractual and written is always a good thing. It is evidence, documentation, and bound by law if done properly.