All bullets can and as demonstrated above, do fail.
The relevant factor is the rate at which they fail in a potentially dangerous manner. For a dangerous game bullet, it is, in my opinion, better to fail by not expanding rather than fail by breaking apart to the degree that they don't reach the vitals.
We have above examples of a failed TSX and a few failed A-frames. Farther up, many examples of failed DGX. What we don't have is a reliable way of knowing the actual number of any of the bullets that failed (the numerator) nor the number fired (the denominator). So we do not know the actual rate of failure for any of these bullets in field conditions.
What we do know is the the Hornady is apparently designed to be more frangible than the other two bullets. We do have some test media results that seem to show the Hornady is more frangible at least in the test media at the tested velocity. It is not unreasonable to conclude that it is not as tough a bullet as the A-frame or the TSX.
Whether this makes it a bad bullet or not all depends on what you want your bullet to do. Is a Jeep Wrangler better than a LaFerrari? Not on a track, but surely in the woods. Define your desired performance, and pick a bullet to match.
If you want a bullet that does maximum damage on a between the ribs broadside shot on a leopard, you would want a softer more frangible bullet than if you want a bullet that reaches the heart from the ass end of a buffalo. The latter bullet will almost assuredly not produce the spectacular result on the leopard that the former will.
I personally prefer (even on plains game) a bullet that penetrates deeply, preferably through the animal. I accept that it will not do as much damage under ideal conditions as a more fragile bullet in exchange for increasing my odds of reaching the vitals under less than ideal conditions. Particularly in Africa where I could end up shooting an eland or a springbok, I want a fairly deep penetrating bullet. Better to put a small hole through the springbok than to break an eland's shoulder without reaching the vitals as far as I'm concerned. On the other hand, when hunting at home on whitetail where one can always wait for the ideal shot, I cannot argue with my son for choosing a soft bullet that makes spectacular kills under the ideal shot condition he is willing to wait for.
With dangerous game, I am even more interested in being able to reach the vitals from less than ideal angles. But, I am planning to use a TSX not a solid at least for my first shot on buffalo this fall. So, I haven't gone all the way to penetration over expansion. Others choose a cup point solid for probably more penetration and less expansion than my TSX. At least one of us firmly supports the DGX. The best information I can find indicates to me that bullet is too soft/fragile for my comfort, but I'm not the one shooting it. If you understand what the bullet is likely to do and use it, you will probably get what you ask for. At this point, it seems relatively clear what the DGX does. Perhaps we should move on to a discussion of what we want a bullet to do rather than continue with this.
The relevant factor is the rate at which they fail in a potentially dangerous manner. For a dangerous game bullet, it is, in my opinion, better to fail by not expanding rather than fail by breaking apart to the degree that they don't reach the vitals.
We have above examples of a failed TSX and a few failed A-frames. Farther up, many examples of failed DGX. What we don't have is a reliable way of knowing the actual number of any of the bullets that failed (the numerator) nor the number fired (the denominator). So we do not know the actual rate of failure for any of these bullets in field conditions.
What we do know is the the Hornady is apparently designed to be more frangible than the other two bullets. We do have some test media results that seem to show the Hornady is more frangible at least in the test media at the tested velocity. It is not unreasonable to conclude that it is not as tough a bullet as the A-frame or the TSX.
Whether this makes it a bad bullet or not all depends on what you want your bullet to do. Is a Jeep Wrangler better than a LaFerrari? Not on a track, but surely in the woods. Define your desired performance, and pick a bullet to match.
If you want a bullet that does maximum damage on a between the ribs broadside shot on a leopard, you would want a softer more frangible bullet than if you want a bullet that reaches the heart from the ass end of a buffalo. The latter bullet will almost assuredly not produce the spectacular result on the leopard that the former will.
I personally prefer (even on plains game) a bullet that penetrates deeply, preferably through the animal. I accept that it will not do as much damage under ideal conditions as a more fragile bullet in exchange for increasing my odds of reaching the vitals under less than ideal conditions. Particularly in Africa where I could end up shooting an eland or a springbok, I want a fairly deep penetrating bullet. Better to put a small hole through the springbok than to break an eland's shoulder without reaching the vitals as far as I'm concerned. On the other hand, when hunting at home on whitetail where one can always wait for the ideal shot, I cannot argue with my son for choosing a soft bullet that makes spectacular kills under the ideal shot condition he is willing to wait for.
With dangerous game, I am even more interested in being able to reach the vitals from less than ideal angles. But, I am planning to use a TSX not a solid at least for my first shot on buffalo this fall. So, I haven't gone all the way to penetration over expansion. Others choose a cup point solid for probably more penetration and less expansion than my TSX. At least one of us firmly supports the DGX. The best information I can find indicates to me that bullet is too soft/fragile for my comfort, but I'm not the one shooting it. If you understand what the bullet is likely to do and use it, you will probably get what you ask for. At this point, it seems relatively clear what the DGX does. Perhaps we should move on to a discussion of what we want a bullet to do rather than continue with this.