You must have missed the Fiasco when he dominated mid lane. (for all the wrong reasons)
They gutted her viability well before the rework. And the Rework failed, cause they centered it around a core Identity that they Really hate. (I.E. The dedicated healer)
The Health battery concept, in theory sounded like a good idea. Except that the 10% HP ratio, on a squishy support is completely detrimental. And since they didn't give her any kind of HP ratio to compensate. their is a strong deterrent from making her tanky. All while her magey aspects are a bit lacker lust. They also attacked her HP regen, which also hit her synergy with regen items. all thanks to the season 5 changes. So the only reliable way for health batter to reliably gain back health is to use an unreliable skill. All while having few tools to actually safe guard said health.