Been farming Mr Nates for days now trying to get a corrosive cutsman to drop, guess RNGesus hates me. For a break, I decided to go farm some other bosses to compare drop rates. For 30 kills on Mayhem 4 I got:
Graveward - 102 Legendaries
Gigamind - 67 Legendaries
Boreman Nates - 11 Legendaries (1 shock cutsman with no annoints)
It probably takes me longer to kill Mr Nates than it does Graveward so the rewards for doing so seem totally out of whack.