Pros: Cheap, good insulator, doesn’t require any experience to install, local, low environmental footprint.

Cons: Thick, does not handle moisture well at all so it must be completely sealed. A small leak will ruin all the hay in the exposed area and, with a bit of airflow, can spontaneously combust. More likely, though, is that it just degrades rapidly. Like a lot of things, good planning can keep you fine.