There is some truth to that. Unions typically focus on seniority above all else so there is no advantage to being objectively better at the job, just don't be so bad you are fired and you are fine. They also insist that if you want to move to management you start at the bottom thus ensuring that someone who might want to change tracks is worse off and in turn there is no reason to try.
Unions are not all bad, but they deserve a lot of criticism as well.