Well there's only two other countries with more than 330 million people so that's a bit of a poor argument to make, and it assumes that the well-functioning systems in (most, if not all) other western countries won't scale - I'm not entirely sure where such an assumption comes from.

Probably a refusal to imagine it as a possibility in the first place, I suppose.

National level public healthcare would be difficult to implement in the US for the same reason a pan European standard would be hard to implement.

That said, if you do it on a state by state basis then qualify will vary just like it does between the different European countries.

But as the US is a single nation, the mere existence of variation will mean some people say the system is a failure.

Because it working in smaller countries means it could never work in a large one?

But, why not? How is population size relevant?

Well "flawless" is obviously a disingenuous expectation on your part.

I would direct you to the United States which has a pretty amazing public healthcare system called Medicare that has an 80+% approval rating among those lucky enough to be on it.