Finding epic equipment in the heroic dungeons is like searching for the needle in the hay. The drop chance is extremely low, I've practically haven't had a drop in months of playing. (Might be my bad luck, not sure about others though.)
Getting all 4 pieces of epic equipment for 50+ units is next to impossible through heroic dungeons.
My suggestion is to implement a mastery system. For example for every 10 successful dungeons, you get a star in that specific dungeon. Which might further increase the drop rate of epic equipment by 1% and perhaps a little bonus in gold and other stuff. The mastery system could be limited up to 10 stars, for every star gained, getting the next star would take more visits to said dungeon.
To make getting a star more difficult, could offer a sacrificial ritual at the oracle as well.