I'm speaking from the experience of being married to a teacher at a very large highschool in a medium sized city here in the US. In all the years of her talking to me about her work I don't think she's ever mentioned her union. Maybe she's just not at the admin level and dealing with it but given the number of vacancies at her school (which is considered one of the best in our city) - I don't even think the union is coming into play. They need staff.
Substitute teachers typically aren't part of unions but schools are forced to keep bringing in the ones that don't care at all because there's just not enough.