Strikes can be good in the sense that the workers and employees are advocating for what they believe is a lack of (proper wages, working conditions, etc). However, the effect of these strikes CAN be quite damaging to the welfare of society as a whole. For example, the Pullman Strike was unsuccessful (despite the fair goals it had) and this caused a nationwide disruption of the railroads and the federal troops had to be sent to break the strike. People ended up dead.
And so, strikes are both good and bad. Good because they bring attention to what the needs of the workers are and can be POSSIBLY bad because of the unknown effects.