The answer to this depends largely on the attitude of the union and its relationship with the management of the firm or firms it negotiates with.
In theory, unions will serve the best interests of workers. They will typically be able to get higher pay and more favorable work rules for their members. However, in the long term, it is quite possible for unions to work against the interests of their members. When a union asks for too much and is too antagonistic towards the firm, it can actually harm the firm in the long term. This can ultimately lead to a situation where, for example, workers get high pay in the short term but then lose their jobs completely when the firm closes the plant to move to a place where labor will be less expensive.