Well, it was used quite successfully on bison.
The 45-70 was introduced in 1873. Bison were almost gone by then and laws were passed in 1874 banning bison hunting to preserve the handful left. I'm sure it killed a few, but in the 1870's 45-70 was considered under powered for game larger than deer. Those original loads were about the same as a 45 caliber muzzle loading rifle, which is the legal minimum for deer in many places and not legal for game larger than deer in many places.
Modern loads are a different story in rifles that will handle them.
I had one of the Marlin 1895's shortly after they re-introduced them in the early 1970's. Added a guide gun later, but sold both about 15 years ago. Never killed anything but one deer. It was dead, but I never really warmed up to the round. I did like the older Marlin rifle though. With milder loads it was enjoyable to shoot. But I felt hotter loads recoil was out of proportion to the performance. If I'm going to get hit that hard I'd rather move up to 375, which has less recoil than hot 45-70 loads.
Not so sure I'd want to hunt cape buffalo with one either. I watched Tony Makris on the hunting show "Under Wild Skies" take one with a 45-70. It took 6-7 shots to put it down and no telling how long. They kept cutting away and trailing the animal then fire another shot etc.
