#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.
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.