I worked in a shipyard that had unions, I didn't join.
We were building destroyers for the navy.
The cost of those destroyers were apx 1/3 more than they would have been had it not been union.
On union jobs, if you need X number of people to do the job, the union will require you to hire not just the X number of people that are needed to do the job, but also Y number of extra people.
The extra cost of people that aren't needed is passed on just like the cost of the people that are needed.
Building Navy ships is not private sector. Apples to oranges.