I don't know what to make of the teachers' unions. My personal experience with individual teachers is that they work plenty, and what I saw during remote school was no exception. (Being a kindergarten teacher over Zoom is hard!) And I never heard a particular teacher in my area advocate for closed or remote schools.
But school administrators blamed the teachers' unions a fair bit for being slow to reopen. In general I suspect school administrators were looking to blame someone else for their own leadership failings, but there still might be some truth in it. I could see how the unions might have adopted the position of the most risk-adverse (and/or laziest) of the teachers.