Are Quickdeath's just too powerful?

So... while looking over TO's Sathar deckplans the idea of having six quickdeaths unleashed on a party of even bad ass spacemarines seems like a quick route to a TPK.<div><br /></div><div>Back in highschool my running of Voltumus never got far enough for the players to encounter the quickdeath.... but just looking at thier stats... they seem like like that are hella dangerous and an easy way to randomly end up with dead PCs.</div><div><br /></div><div><br /></div><div>In your experience is using quickdeaths a bad thing to do if you want to challenge but not randomly kill PCs?  As in... if you run a story/character plot driven game random PC deaths crush your game so I'd be very gun shy to use any quickdeaths.</div><div><br /></div><div><br /></div>