I'd argue there's few Americans I flat out couldn't understand, even if it sounds like they're putting their words through a blender. And I say that having lived all over the country, Northeast, Midwest, West, and deep South. Accents can be thick but they're largely intelligible. Unlike, say, the Scots.

Especially compared to a language like German. I took 5 years of German and still didn't have a damn clue what anyone was saying if they were talking in dialect.