I have taught in situations where there is no union at a charter school in The Villages, Florida, where a stong union exists, and where there is an association or "loose" version of a union. When there is no union at all, teachers are the victims of lazy students, their affluent parents, and derriere-kissing administration--too often excellent teachers were fired without cause simply because the kids didn't want to work and felt they were above it all. Bonuses were given only to those teachers who "partied" with the principal--no, thank you. I actually had a student tell me he would get me fired because I expected too much of them. What does that do to teacher morale, and what lessons are the kids learning? Of course, the principal for whom I worked is no longer there...something about those lazy kids not getting accepted to their Ivy-league schools. Go figure.
Strong unions do protect poor teachers. In some cases, they also support "merit pay" for teachers which I don't support. At any given moment, I work hard to help my students progress but one bad test day or a vindictive student could intentionally sabotage results which in turn affect my earning potential. I teach because I love it, but I have to feed my family as well.
This is a tangled web. Like the welfare system, unions were created with good intentions, but none of it is perfect.