Teachers union(s) are some of the highest profile anti union targets in the US as well. There’s also issues on a structural level that leads to poor compensation for teachers vs other government positions.

Really school funding and public education in the US in general is in a very strange place across the board and has been for decades

Maybe I'm missing something but I haven't seen a lot of teacher unions being broken so I'm not sure what you mean.

The results for a given teacher are poor no matter what the reasons, it's a bad "hey get a union" rallying cry IMO.