Senna is blatantly overpowered. It was only slightly diluted when she first released because of the; 'New Champ Effect'. Her win rate was abysmal for the first week, and has quickly shot up as she's learned.
She has, fundamentally; the most "we'll eat our own words" kit I've seen from the balance team at Riot HQ in a long time.
Her 'Q' is the embodiment of disparity between the current balance team versus the old.
's original 'W': Essence Flux was heavily changed and gutted from it's previous iteration because it was considered; "impossible to balance". This was because it did the following to all allies and enemies it passed through respectively; Heal Allies, Damage Enemies | Buff Ally AS, Debuff Enemy AS.
's current 'Q'; Heals all allies and damages all enemies within, while also healing herself for every ally healed and granting an MS boost / AA boost to herself.
^That's just the tip of the iceberg. It was evident with Kai'sa, it was evident with Pyke, and it was evident with post-Kayle. The current balance team at Riot is lead by [redacted] that listens to [redacted] on creating overloaded Champions with kits that sell. Income has become Riot's primary balance function.