I'd say a proper union is efficient and useful enough that employees who are free to choose whether to join or not mostly choose to join of their own free will.

I'd say a proper union is efficient and useful enough that employees who are free to choose whether to join or not mostly choose to join of their own free will. If you have to force people to join, then maybe it's not actually that great. Or alternatively, it has less incentive to stay efficient and useful and police itself if everyone is forced to join.