Consider Obamacare. Objectively, it was an awesome law that covered millions of previously uninsured Americans. However, it was a PR disaster because people were told they could keep their plans and their premiums wouldn’t increase. Because of how impossible it was to lie about those two things and because the Obama administration lied about them anyway, people greatly distrusted Obamacare, even though most of them actually liked the law and its outcomes.
Similarly, if a worker is told by a pro-union advocate that no one will lose their job with a union but that worker also readily sees that many workers lose their jobs when unions cause companies to collapse, that worker is going to feel lied to despite knowing that union membership is probably in their best interests.
I think it’s pretty silly to quibble over whether someone will answer gotcha questions like “can you name one good/bad thing that has ever happened in a unionized/non-unionized workplace.” That’s just not relevant.