I don't understand the argument.
A conical bullet weighs nearly 2x the weight of a roundball and in some cases more than that. The muzzle loading rifles handle them quite nicely. A round ball that weighs, pick a number, 150 grs with 3 #4 buck pellets rammed nicely with an over shot wad so that they don't escape and cause a barrel obstruction should be of no more danger than a conical of the same weight. The "buck and ball" loading has been around for a LONG time. I have never seen anyone claim any extreme effectiveness for the load. It was apparently a defensive load rather than offensive/hunting load.
Let's don't get excited and run in circles unless necessary.
Rifleman 1776 just sent me a PM with a method of getting hurt with the 2 ball load. I would never thought of what he suggested. If there is a way to screw something up, someone will find it.
With that knowledge, I'm forced to put myself in the camp of don't do it!