Not sure that this is still done, but it has been the case that if something goes wrong with a Disney poncho, any shop selling them will trade it for a new one.
I had a couple of the side snaps come off one I'd had for a few years. Took it to our resort's gift shop and asked for a new one. CM took a quick look at that one and handed me a new one, no questions.
The Disney ponchos are very durable, as stated above. We have one we bought almost 10 years ago that's looking a bit yellowish but otherwise still in great shape.
Definitely worth the price, IMO.