I've only been using Pecards for about 35 years, so can't comment on what it does long term. I was given a box of military slings that were folded and tied many years ago, and appear not to have been stored very well. They had dried to the point that they would not stand trying to straighten then out. I applied Pecards in repeated coats and the slings eventually relaxed enough to be used on rifles, and while I would not trust the weight of a rifle on them, they make good display slings. Pecards is the only thing I use on leather that needs treating.

The worst thing I ever used was Lexol. I had a Model 1909 holster that was very dry, and applied repeated coats of the Lexol. As it dried the leather actually got more dry and brittle, rendering the holster useless.

Ballistol is mineral oil based, so should do no more harm than Baby Oil. The mineral oil will turn the leather dark.

Personally I have never had the white powder form on Pecards treated leather, but have a couple of slings that I treated with Mink Oil about 35 years ago, and to this day they still form the white substance. The slings are still supple, and the white substance wipes off with a soft cloth.