I've seen some heated debates about he XTP's on several forums. More than any other bullet for some reason. One time they work great on something big like a Griz, next time they fail on a small deer. I don't know the answer because I don't use them but I've read it.
My guess would be either too much velocity, just like with a cast bullet, or hard bone, same with a cast bullet. But again, I'm just guessing. Don't see the arguments with Speers or Sierra's. Back when I shot some Sierra's they seemed a bit on the hard side in some bullet weights but very accurate!

Dick