Has a lot to do with flexibility... Same goes for any 'awkward' exercise. Ever notice how you can be in great shape, but you try something new (hence awkward) and it kills you quick?
Burpees are awkward for most at first, but for a guy like me (tall and bad lower body flexibility), they're always going to be hard, even after practice.
Some chubby little long torso Asian kid who can literally squat atg will smoke me in a burpee competition any day, even if I could lap him in a mile race.