Recourse to a strike is morally legitimate when it cannot be avoided, or at least when it is necessary to obtain a proportionate benefit. It becomes morally unacceptable when accompanied by violence, or when objectives are included that are not directly linked to working conditions or are contrary to the common good.
Unemployment almost always wounds its victim's dignity and threatens the equilibrium of his life. Besides the harm done to him personally, it entails many risks for his family.
Source: Catechism of the Catholic Church (Vatican.va)