Imo, Flay’s Blood Eruption should occur if the enemy dies within 2 sec of the Flay hit and with Plasma Fountain node maxed it should occur every 1st time you hit a rare or boss enemy and not every 2nd time. And it’s not like it has any serious damage output the way it is right now.
I use it as my movement skill it feels like it’s underpeforming.
At the Season 4 DPS tier list of Maxroll builds, the Flay build was in second place, behind only the (eternally broken) Ballista Falconeer build.
Interesting. I think Flay is one of the best Acolyte skills right now. Giant versatile tree, so many ways to play it and all of them feel very good.
Could you post some example characters or builds that feel bad? Maybe we spot something you are missing.
I am definitely getting soft in my old age as this is second time i am agreeing with @Heavy on the same day 
. Apart from the issues mention by @Heavy one of the best things is that the mana consumption is well balanced so that, if you are careful and judicious in using it, you can sustain it. If you are reckless and constantly spamming you will run out.