I don't know...I think the people losing animals with a .243/6mm would lose them with about anything as they're making garbage shots.
My first big-game rifle was a 6MM and I've shot it and a .243 quite a bit at big-game. Reviewing my field notes, I've shot 27 pronghorn, 24 deer, 3 elk, 1 bighorn, and 1 black bear with .243/6MM. Not a ton of big-game, but enough to form the opinion that the .243/6MM is a capable round.
I shot about half with 100 grain nosler partitions, and the other half with 100 grain old style nosler solid base bullets (lead tip) and recovered very few bullets.
In fairness, I did lose one whitetail buck. It was 100% my fault as I shot it trotting and hit low in the brisket and busted the off-side leg. I tracked the buck through the snow until it eventually dropped below the snow-line and never did find it. I was about 17-18 years old and made a bad choice on shot selection and shouldn't have shot at all. Definitely wasn't the caliber/rifle...it was all on me and it wouldn't have mattered what I shot it with. A poor shot is a poor shot.
IME, the three most important things in killing big-game are, shot placement, shot placement and shot placement.