#2 Very niche
Whether The WWE Universe wants to admit it or not, The Fiend as a whole is one of the most niche characters in the company's history. Of course, some will point to his Universal Title run and his early undefeated streak as proof of that not being true, but one has to remember what happened once that started to subside.
Unfortunately for The Fiend, his popularity took a hit after he got switched to Friday Night SmackDown. That's supposedly what lead to Vince McMahon putting the title on Goldberg after he popped a rating on a throwaway episode of SmackDown and paved the way for what could have been the end of The Fiend.
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That's the biggest problem with The Fiend character as a whole. It's the fact that he couldn't hold ratings for that small period of time and forced WWE to go in another direction. In the end, that likely means he will remain a very niche Superstar for years to come and should probably not be featured in a title program for long periods of time.