Is it a problem that unions have the power to stop the work for safety reasons? On the contrary, I believe that this is a
necessity if you want to preserve the value of human life under dangerous work.
I am currently reading about the history of the North Sea subsea workers during the great petroleum expansion from 1960-2000, and the safety culture which followed from American drilling companies was just atrocious. Divers would be forced into lethal working conditions in situations where it was completely out of proportion to the importance of the work. 16-hour shifts under water, aggressive decompression tables which frequently led to both acute diving sickness and long-term delibitating injury, in addition to frequent accidents. (An "accident" under water usually means death). One time they sent down a guy who had been picked up from a pub three hours prior, who started the dive by puking in front of the whole crew. He died.
So tl;dr, giving unions the power to temporarily stop the work for safety concerns is vital if you want to make sure that the workers mostly get home safely.