We have had huge huge issues with this.
The worst was when H had a surprise triple bypass two years ago. Obviously the bills were astronomical, we shot past our OOP max in a hot second. But for some reason, the insurance company just would.not.pay this one bill for about 1000$. It was tiny compared to the total cost of his surgery and hospital stay (total negotiated amount was about 200k iirc). But for whatever reason, they wouldn't pay it.
So I played this back and forth game for months, calling the hospital, trying to get an explanation for what it was, what was the billing code, could they possibly bill it as anything else, then calling the insurance company to try to get them to fucking pay it, then calling the hospital back to say I was working on it and please don't assess late fees or send to collections. Repeat over and over and over and over. It was my first time going through this, so I didn't realize until after the fact how much they were screwing me around. Finally after about six months of this I called the insurance company and wouldn't get off the phone until I spoke to several levels of supervisors up, got a direct phone number to call them back, and then called me directly, daily, until it was resolved. Once I had a direct number of a higher up person, and I bugged them, it magically got resolved. Fuckers.
Now I know better, and just do that in the first place (and good tip from the PP to invoke a complaint with the insurance commission!) things go more smoothly. But I imagine most people give up well before they ever figure that out, and just pay the bill so it doesn't go to collections.
Anyway, they also try to screw you over in all kinds of small ways. Every year we have to send in a form saying we don't have any other insurance. We send it in, but for each person in the family, the first claim of the year, they just deny it. Then we have to call them and tell them verbally to pay the fucking claim because no we don't have other insurance, bastards.
Or they have this magical seperate mental health coverage that is in tiny print, and has a totally different set of in network providers. So we go to a clinic that is in network for everything else, but since we took my son for a mental health eval, nope, not covered. 3k out of pocket. (In that case luckily there was a state mental health coverage for children program that did pick up the cost, but it was a one time deal, so if we ever need to use it again, welp, too bad.)
Or once, a hospital sent us a bill, insurance won't cover. We call, much confusion ensues. Finally we figure out, it wasn't a bill from 6 month ago, it was from a YEAR and 6 months ago. They were billing the wrong insurance. But oh, the old insurance won't pay either. And basically there is no time limit (in my state anyway) for how long they can take to bill you. That one the hospital eventually wrote off after I fought it and fought it.
So, long story short, universal health care and death panels for everyone!