#2) Demerit: Unnecessary involvement in storylines.

John Cena's open challenge was particularly great because it had no effect on the ongoing storylines. The matches were completely one-off and provided PPV level matches on weekly programming.
However, recently WWE brought back the US Open Challenge during AJ Styles' run which was hampered due to the Corbin-Dillinger-Styles rivalry. This decision was not loved by many fans because it took away the novelty of this open challenge.
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If Reigns' open challenge pans out the same way, the whole Open Challenge concept will become useless and would turn into a tool for the main storyline advancements.
How WWE has messed up John Cena's last run - Check here!