Any piece of metal injected at high speed into the heart/lung area of any animal has a high likelihood of killing that animal. It's not so much a matter of can a bullet ever do it's job, it's a question of reliability, will the bullet do the job shot after shot after shot. No one hunter (not counting PH's) is likely to shoot enough animals to answer that question. But thanks to forums like this, we can gather enough statistical data from a large enough group of hunters to answer that question.
When it comes to the DGX, the answer to that question of reliable killing has been answered and that answer is a firm no in my opinion. It doesn't mean it doesn't ever work, it doesn't mean that it won't work perhaps even most of the time where most means more than half. But it has performed poorly in too many instances for me to even consider using it especially with the availability of what I consider to be far superior bullets.
Getting too attached to a bullet because it has performed well for you a few occasions seems unwise to me too. I've killed plenty of animals with the Nosler Partition. Love the bullet, love the story that drove John Nosler to develop a better bullet. But as much as I love the bullet, I would not think for a moment of using it on Buffalo. I would bet Mr. Nosler would consider the A-Frame and NF Bonded cores to be the natural evolution of improvement of the Partition design.
Now as to why the DGX has seen the wide spectrum of performance, I don't know. I can think of a number of possibilities from poor design to poor quality control in manufacturing. But that's for the Hornady guys to figure out and I hope they do. I appreciate what Hornady has done for shooting and especially providing brass in some of the classic calibers. But the DGX is not a product I have any intention of buying.