#1 It's lore
Believe it or not, the lore of The Fiend is by far its biggest weakness. In fact, between hinting at a true weakness of The Fiend and then not following up on it, and the acid trip style of The Firefly Funhouse match, it's hard to imagine what the future of the character is.
If nothing else, WWE has introduced a lot of different elements of The Fiend character, but sometimes forgets to keep them into the narrative. For example, Ramblin Rabbit was swearing up and down that The Fiend had an exploitable weakness, but that was never followed up on.
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In the end, it was a very jarring moment in the lore of the character and begs the question of if it will be implemented again or not? Unfortunately for The WWE Universe, once a character's lore is changed or goes unacknowledged for a long time, it becomes harder to really get behind the gimmick as a viable threat.