In reply to abcdef: pretty steep both times, but I suspect any fall on techy III or IV and above would snap the leash, they are usually made out of pretty flimsy material, certainly mine is more a covering for the elastic than of any structural assistance. Both times the middle of the leash snapped, about 1/3 of the way along one of the umbilicals.
There is a video here of the first one I was referring to, skip to 3:30;
You can see the leash snap immediately before the tool placement rips, and the tool placement was only thinly in pretty thin ice, not what you'd want to belay off anyway. I suspect in any kind of average mixed route the placement would be much more secure, thus still snapping the leash.
I suppose it depends by what you mean by "average angled terrain", I'm talking IV-VI, but equally on easier stuff I still think the leash would fail 90% of the time before the tool ripped (assuming its in an actual placement).
Agreed its probably less likely to impale you than wrist loops.
Back to the OP, I forgot to mention that I traditionally don't use them on ice since I can just whack the tool in anywhere and let go, whereas on mixed the placements often need weighting all the time.