I feel the AirPods are built very durable. They're lightweight enough it wouldn't damage the plastic casing, aside from what could happen to the internal components. But ultimately, they don't feel cheap or poorly constructed, which are bonded well with the adhesive.
The Plastic seems of that of a higher grade than what other manufactures might use for earbuds.
I have not heard of anybody breaking their AirPods as of yet during a fall.