More like the thickest. Black Rough-Out is the same as black oil tan aka "black smooth", but used the other way around (with the rough flesh side facing outwards). It's among the thickest, strongest leathers White's is offering, and the default leather for all their "real" workboots.
I think Break-in depends not so much on the leather type as ist depends on correct fit and sizing. If your new boots fit well, there will be almost no break-in discomfort. If something's not entirely right, however, even the soft leathers like Chromexcel can become painful.
Maybe having the smooth side of the leather towards the skin feels better on the tight spots during break in?
The break in for my boots wasn't bad at all, except for one spot. It was where a bone on my foot sticks out right where the vamp is reinforced. I used that 50/50 water alcohol boot stretch mix to help the leather mold to my foot. The store that measured my feet gave it to me free after they stretched a pair of my dad's older cowboy boots. I don't think it "stretches" the heavier leathers as much as it helps mould the leather around your foot. I had better results putting it on the inside, by the way.
Not sure if I had talked about that before.
edit: Forgot to mention, my boots are the Black oil tan Packer model. Pictures are somewhere in this thread.