This is easy Archie.

A union shop has to give the next minimal union person the next job.

A non-union shop can give the best qualified person the next job (who might not be union).

Job qualifications often are erased or minimized by union membership. Bust your butt doing the same job that someone else is doing at the minimum to get the same pay only encourages lazy workers to join unions to get work. If you want to fight merit pay for work done, join a union.

Any questions?
Bob