It's amazing how many people hate United Health. If you ask any random person on the street, they'll have a story it seems. I have a coworker who is currently fighting them for approval of $20,000 in claims for her husband. He also needs surgery, which United Health won't approve.
When we had United Health at my previous employer, one of my coworkers got stuck with $12,000 in bills. She ended up selling her house to pay it off.
My personal story with them is that I found a breast lump and bleeding. My doctor said I needed a mammogram so we did that, not thinking it would be an issue with the insurance. That was followed up with an ultrasound. Both were inconclusive and the doctor recommended surgery. They denied all of the claims because I was a year younger than they allow payments for those tests. I ended up paying for the tests myself and decided not to do the surgery. When I got different insurance, another set of mammograms were done, and they didn't find anything. It's been wait and watch since then.