... until you get a really expensive chronic disease. And then it fails miserably.
I know upper middle class parents whose kids got a disease which need ~$5000/month meds for a few years (and nontrivial infusion center costs on top).
It worked ok for some, many had been informed that the insurance company decided not to pay - requiring lawsuits (some settled, some ongoing).
No European, Canadian or Israeli parent that I know of kid with the same disease ever had to fight or pay out of pocket - it was all covered by the single payer with no hassles.