I bought a Buckstalker probably 20 years ago when they first came out. For my setup, I only shot the Thompson Center Maxi Hunters, 275 grain bullets.
I took 3 or 4 deer with it.
I don't know what, if anything is different with today's model Buckstalker or why they only recommend saboted bullets.
Under certain conditions, a lead conical bullet can walk off the powder charge. Especially if you're carrying the gun around the woods. You can bump and shake the gun without realizing.
When I'm hunting, I will take the ramrod and push it down the barrel to reseat the charge. Just to check. Once in a while, I have felt that the bullet has indeed move up a tiny bit.
When using sabots, I don't think they ever moved.