The closest thing I've tested to a failure on a variety of mechanicals is when the blades shear off on some of the cheaper (thin-narrow blades) ones. I use an old "block" tarket to test mine. If I can't put the head through the target multiple times without loosing blades or causeing a major failure, I won't use it.

Shot placement is the biggest thing. I also know of a buck that was shot this past season with a fixed head at a hard quartering-away angle, and the broadhead deflected off the ribs, basically going through the arm-pit without entering the rib-cage of the buck. It left a major wound, but the buck survived and was killed during gun season. Would you consider this "head failure"? Anything can happen with any head. You don't believe me, how would you explain lost animals before the advent of mechanicals?

I use mechanicals because it reduces my tuning time, increases my forgiveness and accuracy at greater distances. I've killed something like 12 or 14 different deer with my heads only loosing one to a low brisket shot, which wasn't the heads fault - it was only following my aim!