External polystyrene panels have been used in Europe for over forty years and are pretty widespread so I'd say their long term durability must be reasonably good. I did a Google search but nothing came up about any statistics on it though.
If you get hail that big I think the only external cladding or roofing that would stand up to it would have to be 150mm concrete ! I wouldn't like to be caught out in that, that's for sure.
Stewie