I have no problem with more difficult dungeons, especially if they provide better rewards. Problem becomes when you throw that dungeon into the same pool as the remaining dungeons and place it on equal footing. CoEN is way more difficult than any of the other dungeons, the reward is not superior, and it pretty much turns into an instant dead key above 10+.
If CoEN was separate from the M+ system, then fine, make it as hard as they want. Otherwise, balance it according to the other dungeons.