Most of his kit revolves around his passive that isn't really that good.
At the passive's max rank, with max stacks, and the full duration, the target takes 216 true damage.
Vayne's W, Silver Bolts, does 60 true damage + 8% of their maximum health and in the time it takes for Twitch to apply the max stacks of the DoT, Vayne with the same attack speed could do 120 true damage + 16% of their maximum health.
Twitch can potentially cover an entire team with this, but It's insanely difficult and not really that rewarding for how difficult it can be.
Twitch's passive doesn't even do damage early game really since it just stops or even lowers their health regen.
I feel like Twitch could be buffed on his passive's overall damage and turn him into a late game hypercarry he should be. He's really strong late game, but what's the point of playing him in solo queue or picking him over Tristana and Vayne.