Did somebody say “RNG”?
Other players aggregated Explore run statistics suggesting:
Combined, this is a 0.025-0.1% chance of getting a Hoard Mimic per Explore Boss defeated.
0.025% is better than the Mythic drop rate from Glory chests (0.01%).
0.1% is same as the Mythic drop rate from Gem Chests.
Math aside, the major difference is the sheer amount of player time required to actually reach the Explore boss for just 1 chance at the Boss Chest, vs. acquisition of Gem Keys + redemption in bulk.
As such, I certainly wouldn’t mind a higher encounter rate of Mimic Bosses and Hoard Mimics inside that (say, 15% each?) to slightly offset the sheer time and effort involved in farming higher level Explore runs.