This is an interesting article: Medicare For All Would Be Difficult
This is about "surprise billing", that happens when an emergency makes you seek medical care outside your medical group or HMO. Some of the bills you get are many times what they would charge someone in their own plan, and you are stuck with them.
Hospitals and doctors have fought tooth and nail to keep this scam intact, but they lost out in California in 2017. Medicare For All would end this, because it would pay anybody the same amount for a service.