My experience goes like this:
Dip dye a scrap piece in Fiebings Pro Oil dye (saddle color). Let dry. Blotchy. Tried 2 more times with different leather from different cuts. Same results.
It must not happen to everyone because a lot of people use the stuff, but I will say, Angelus and Lincoln dyes have NEVER done this and I've applied them to crap leather when I made the kids in the neighborhood holsters for cap guns. The holsters all came out uniform in color, as always.