I've a Rocky Mountain Vertex Team Scandium (Easton Scandium main tubes) and love it. In terms of its strength and rigidity it has totally coped with anything I've been brave enough to throw it at. Although its lack of weight makes it a pleasure to ride one of the, surprisingly, really nice things about it is how easy it is to portage, lifting it over stuff never fails to put a smile on my face.
I think we have to assume that for any given discipline; XC, TT, DH, Track etc, it's highly unlikely that a frame made by competent manufacturer will fail under normal use. Scandium frames are no more likely to fail than any other material if the frame is well designed and built. But then, my mate's custom built Merlin developed a crack around the seat clamp. Which raised a few eyebrows I can tell you.