I don't know where it stems from, but I think it's harder to fire people in union shops. In a non-union shop, you have to work with the person, their manager, and HR to see if you can correct someone's performance, and if that does not work, let them go. At a union shop, I imagine someone from the union will also be involved in this process, and it's hard to imagine how this process would be made easier through the involvement of another person, who happens to hold a bias.

You're forgiven, but not correct re: me being under-qualified to comment. I have interacted with union shops and non-union shops and have 25 years of professional experience, so please don't be dismissive.

I've worked with union shops as clients and I've hit a surprising amount of friction due to things which seemed union related. Things like "that's not my job and we need XYZ to do it" and what seemed like people slacking at their job from the outside. I once needed a desk moved a few feet which required a power strip to be plugged into a different wall outlet, and was told we need to wait for the electricians. Risking the wrath and fallout, I did it myself.

I've also seen this kind of behavior at non-union shops, but it was less explicitly stated and it was relatively easy to do something about it.

I'm sure you're correct about the quality of life at union shops.