They can't? How do they force everyone to pay state/local taxes? I doubt states would find it difficult to enforce participation.
On "unbridled state to state migration", I already covered that. Maryland would have to face the real cost of providing universal healthcare, and residents would have to decide whether it's still worth it to live there. Many sick people would decide that it's not, and choose to stay put in Virginia.
And if it truly wouldn't work, then it begs the question: Does it really work on the federal level? Why are costs skyrocketing and providers leaving exchanges?
Maryland would have to face the real cost of providing universal healthcare
The point is that they would have to face much more than the real costs of providing universal health care because they would have to provide care to a lot of people that never paid into the system due to migration.
There's a good suggestion for dealing with this above: https://news.ycombinator.com/item?id=14556688
A US state can't do this on it's own because they can't force everyone to participate and they can't reject entrants from outside. In the Virginia/Maryland example Maryland would face an exodus of healthy and productive people who want lower taxes and an influx of sick people who want the free health care. It's not about "remaining equal in all other categories." It's that it wouldn't really work at all.
That's why you can't really have a system with massively different social services at the state level: the unbridled state to state migration.