I have never even seen a Mythic, and this season did do a lot of boss runs even. I did not even know a treasure goblin or Hordes could drop one. For me the drop rate must be 1 in a million. What I find even more frustrating though is saving up to do something like 30 Durial runs and ending up with not even a single useable item or upgrade. Now I just do the boss runs to pass the time between events or when nobody is doing Pit, Citadel, etc.