It works pretty well. 23% of Americans use Medicaid. (which means it surprisingly covers more people than UK's NHS) About half of all births in the US are paid for by Medicaid.

The Americans you hear about on the news that don't have healthcare are people who have incomes above the eligibility threshold to qualify for Medicaid, or are too young to qualify for Medicare. It's the people in the middle who fall through the cracks in the US.