I've never had problems with the FTX effectively killing game, and never noticed any failure to expand. The tips are fragile, but they are also incredibly large. Rapid expansion is the entire design goal of any tipped bullet, it has a "driver" which starts the expansion by translating meplat contact into expansion - especially in the FTX design where the rubberized tip squishes outward, translating axial impact into radial expansion by elastic deformation. Bottom line, they open up, and they kill deer. Even if you somehow did get a failure, they'll kill just as well as a hardcast, which kills very well, as proven by almost 150 years of field use.
I honestly don't believe folks talking about "fragmenting" of the 325FTX, unless they ran it out of a much larger case than the .45-70. I do believe the bullet will shed weight, meaning it'll lose most of the mass off of the tip/ogive, and I could be convinced someone could hit something hard enough to cause a jacket separation, although I have NEVER seen evidence of it. The tip sheds long before it starts peeling the shank, so I don't think jacket separation is real for that bullet in the .45-70 either. Even if it separates, you're talking about a massive core, a lead slug, so it's still going to kill and still going to penetrate. Since the tip is rubberized, it doesn't drive straight back and compromise the core like a poly tipped bullet, and the core is too massive to simply be ruptured and fragmented. It's kinda like trying to crush a bank building with a semi-truck - you don't have to worry whether the fenders and side mirrors survive, remember, you're sending an 84,000lb hunk of steel at your target. The 325 FTX will kill deer cleanly if you place it well.
Also remember, for a guy to have an actual claim about bullet failure, either failing to expand, or fragmentation, they have to have recovered the animal - meaning the bullet did its job, and now the guy is still whining... most bullet failure threads I have seen end up with the guy admitting they didn't recover the animal, and only speculate it was a bullet failure, because they swear they make a good shot...
4198 has been my go to powder, it and varget, for the 45-70 for many moons, including loads for the 325 FTX. I've been satisfied with their leverevolution powder as well, but since it's a rather limited application powder for me, I usually load 4198 instead.