I came across a house that was built in circa 2002 - the cladding is plaster over polystyrene ...

What do you make of that? Sounds very non permanent to me! 25 year life span? Would it have been lined by building paper with a cavity before the layer of insulation?

Ok I think I found the answer - apparently this very type is the leaky building syndrome type.

Would (not that I'm considering it, just for my education) something circa 2000, plaster over hardiboard, building paper, insulation - but no cavity between building paper and insulation - that would be slightly less prone to weather tightness issues? (Of course depends on a multitude of other factors which all compound but just comparing the cladding system alone)