The fact that they have it on this blatant of a propaganda poster means that unions work.

Not necessarily.

A poster this blatant means unions are bad for management.

It doesn’t prove that unions aren’t bad for both workers and management alike. Business isn’t a zero sum game. To show that something helps workers, you need to demonstrate that it helps workers.

Which is to say, this poster is a bad argument for unions. The success of the writers strike, on the other hand, is a good argument of how unions protect workers from the bad deals management offers.