What are some of the principal reasons for the decrease in the size/number of labor unions?

Majhoshev (2017) (Links to an external site.) explained that the right of workers to form and join trade unions is one of the most important international labor standards. That said, union membership in the United States continues to decrease.

What are some of the principal reasons for the decrease in the size/number of labor unions?

What are the advantages of a labor union in today’s globalized, competitive environment?

What are the disadvantages of a labor union in today’s globalized, competitive environment?