The cornerstone of the case against unions isn't that we should stop respecting people, it's that unions are often no longer the best way to accomplish that goal.
Not to detract from early labor movements, the fights for basic workplace safety and overtime in the twentieth century and those people standing up to violence and harassment for basic human rights were heroic.
I'm glad we had unions, but they also have a dark side, where they can function more like a corrupt cartel over time.
Even without Hoffa-style criminality, they can become unintentionally, institutionally abusive. I've worked blue collar jobs in the rural US and had coworkers rant about their family getting eaten alive by the system. They'd explain how the unions would abuse their new probationary employees by making them work triple shifts in order to drive them away from the profession. I've known career extras desperate for a SAG card basically excluded from doing what they love because the slots are capped, and they're aging out of their passion while they wait their turn. Paying your dues can have a few meanings for people in those jobs. That's even setting aside how it creates steep power differentials that become a magnet for predatory gatekeepers like Weinstein.
I don't feel like typical pro or anti union positions do justice to the complexity here.
I think you have to acknowledge that unions have put a spotlight on massive social failings. But I think we also need to recognize their pathologies as well. Generally we need to move those solutions into less partial laws and courts, instead of creating another unchecked center of power over workers.