This is nice because it's completely flexible/dynamic. You don't have to worry about declaring font sizes for every screen width or breakpoint. That would require an unnecessary amount of media queries...
Well viewport units are completely flexible/dynamic [1]. That's their purpose. But I can see how this library might be a quicker solution in many cases.
Very true. Unfortunately, the support for viewport units is somewhat lacking. Some browsers still require javascript to "repaint" the elements after a manual window resize. It's only a matter of time though, when these JS patches won't be necessary...can't wait.
[1] http://dev.w3.org/csswg/css-values/#viewport-relative-length...