I'm a bit mystified about my Dad's situation with this.
He switched (or "got switched" more likely) from traditional Medicare to a Medicare Advantage plan about four years ago. I think it was a "helpful" phone rep from his insurance company.
What little I understand about MA is that it is similar to an HMO - the federal government gives my Dad's insurer $X per year, and the insurer is supposed to administer his health insurance for the government. Obviously with this structure, the insurer is incentivized to try to deny claims or offer a limited network.
Neither of those has happened with my Dad AFAICT. He has had several expensive surgeries, several hospital visits (with the attendant imaging and lab bills), several ambulance rides, some durable medical equipment, a two week stint in a SNF for intensive PT, a fair amount of home health care visits and physical therapy, as well as several maintenance meds during the past four years.
In all cases, there were no unreasonable delays in care, all the providers have been in network and are not the "fresh out of med school" types, coverage has not been denied for anything, and my Dad's OOP portion has been, in my opinion, ridiculously small: $300 copay on a solid five figure surgical procedure this summer, $103.04 copay for an ER visit, $15 copay per visit for his home health FNP.
We haven't needed to do much pre-auth or gateway visits at all. ne exception was trying to get him a power chair which was like pulling teeth and took 3 months, but that seemed to be the Medicare rules and not his particular insurance situation because it was driven by the DME provider. And all that work didn't really save the insurer much, because he only rented the powerchair for about five months.
He does also get a "window dressing" benefit of $50 to $100 per quarter to buy medical supplies, which I order for him on a website and gets shipped to my door.
I'm sort of waiting for some shoe to drop based on what everyone else has said about Medicare Advantage, but so far it hasn't. He's sort of in a palliative care mode now and may go on hospice sometime in the near future, so maybe he just dodged a bullet or somehow his particular Medicare Advantage plan in our state is somehow good or something.