Why would I pick Cassiopea over Viktor?

has more burst and sustain damage. His stun is more reliable and he can hyper carry while bringing a reliable AOE stun. His early game is just as effective while not being a mana hog. He can make great use of just like . The only thing Cass offers is a highly unreliable W grounded. I can't even say its a better zoning tool as Victor W does a great job too.

Why would I pick Cassiopea over Viktor?

is not nearly as reliable and the rewards of getting her combo right are not that great. Her E dps is poor mid-late game. She isn't that great as a control mage. Her ult makes her vulnerable in close range because any random movement from the target can greatly decrease the chance to stun.

Its like she is stuck between a hyper carry and a control mage and does both jobs poorly.