I hate UB mill. Not because it's bad, but because it seems every time you see a UB deck, it's mill. Like always XD.
The worse thing you can do in any mill deck that folds to a shuffle card. Which seems to be common. I have a playgroup of guys who play together and one of them ran UB mill. I just threw
Kozilek, Butcher of Truth into my deck and, for the longest time he'd scoop because of it. Other cards I use to counter it are
Wheel of Sun and Moon (favorite one). Have at least one other condition, because mill isn't that hard to counter is what I'm saying.
A good card to include is
consuming aberration. That mill ability gets pretty harsh.