I'll try to answer some of your questions but like Mike said, it really depends on the tree. Bear in mind also that the type of tree I usually cut is different from yours.
I'm not much on fancy cuts. I use a humboldt face almost exclusively but I'll vary the bottom with a straight snipe or an angled one if need be. A lot of what I cut has most of it's weight down low and you can't depend on top weight to swing the tree. You have to get that torque into the bottom as soon as the tree commits or all the fancy cuts in the world won't help you.
I've used the sizwheel and the soft dutchman but mostly out of curiosity and they didn't really work out that well for me. More practice with them might be the answer to that. I'll bore leaners if I have to but I prefer the CoosBay.
I've had good luck with sticking a busted off piece of the face into the undercut and using a tapered hinge at the same time.. And wedges...I'm a big fan of wedges and I'll stack them if I have to. On the really big wood I'll use a set of jacks or maybe just a single depending on the situation. Plus wedges.
Failure rate? Define failure. If a tree goes a little out of lead occasionally it's not the end of the world. Most of our skidders are grapple so a crooked lead here and there doesn't make a lot of difference. If we're working short ground a good lead is more imperative but on long ground or gentle ground it's not as critical. I try to keep everything laid out neatly when I can but if things go bad I don't lose any sleep over it. By the same token, if a faller can't keep a lead, ever, he probably won't be around long.
Failure to me means busting up a tree. Other than hitting the bosses pickup it's the worst kind of damage you can do. When you're following a Timbco and cutting the stuff it won't handle you can get into some really big and really valuable timber. Same thing on a yarder or a helicopter sale although a pure lead isn't as critical on those. If you splatter some little 24" or even a 30" you'll get grumbled at but it happens, and the bullbuck knows it happens...so no big deal if it doesn't happen too often. But if you toothpick a 48" or a 60" pine or cedar or doug or, God forbid, a Redwood you'd best have some kind of explanation ready that makes sense. You don't get away with many of those without getting an early trip to town.
Maybe what I'm trying to say here is to use what works the best for you. And if you lose one now and then, big deal. It happens. It even happens to us old guys with gray hair and wrinkles.