It can’t do a loop (or a cusp, in the limit) like cubic Beziers can, and may instead "lock" to a weird hook-like shape, I’m not sure whether it’s just an artifact of the way the demo tries to match the curve to a Bezier.

Loops and cusps are of course not very useful in 2D graphic or typeface design, and even if you’re animating a plane or designing a rollercoaster you’d probably use more than one Bezier to make up a looping shape.