It's been a long time since I ran across Doc. I believe I first met him at Larry Twymans' rondezvous back in the early '70's down in Misseri. Him and Wayne Bloomquist were drinking whiskey, singing, "Together, Again," by Buck Owens, I believe.
Still buy powder from him from time to time.
He made kind of a broad statement there. Antimony will work against the round ball accuracy, and must be a factor from the beginning of load development. I shoot mainly WW's for round ball, and pure lead won't shoot any better in my rifles. I believe what he meant was, if you change alloys, you will have differences in the action of the powder pressure upon the ball.
Also, when you begin adding elements into an alloy, the amounts are very hard to control and without weighing each ball you will have a greater variation in weight, although it will only be a few grains. For a plinker or hunter, it's a non-problem.