The Flash: 10 most disliked storylines, ranked
9. Cicada's reign of repetitiveness
Cicada's monotone reign of terror felt like it went on for an eternity and it really spotlighted The Flash's struggles to maintain its momentum for a full 20+ episode season. It was an issue that reared its head in season 4 when DeVoe needed a bunch of metahumans (and villain-of-the-week episodes) to extend his own arc, but it was far more obvious in season 5.
I think the reason that the fans were more vocal about this one was the fact that Cicada was on-screen for most of the season (while DeVoe's absence could be explained away in filler episodes). He would show up, mumble about how this would "end now", lose the battle, and fly away with a groan. It happened every week, diluting the villain further and making the show's heroes look stupid because they repeatedly allowed him to escape.
Cicada also wasn't a great villain, so the moment the Reverse-Flash subplot was introduced, he completely lost our attention. And yet the show continued with the arc right through to the end of the season. Cicada may have told us that "it ends now", but my goodness, it never ended with him.
The Cicada plot wasn't offensive. It was just dull, and it remains one of The Flash's most infamous, simply due to the fact that it went round in circles for far too long.