Zilean' damage was already unreliable
Things that could be used to counter play zilean bombs pre-rework:
Spell shields Shields Heals Troll poll Troll pool Rappell Summoner heal Cleanse QSS
So why did it need to be a skillshot? If you're going to make it a skillshot, take it off of the timer delay. Making the damage a 4 second delay AND a skillshot prett,y much makes a zilean bomb the least reliable source of damage in the entire game.