Riot, I'll just over-simplify all the problems with Cassiopeia's Rework. "She's too brutish with E"

"LOL I'M DOING 600 DAMAGE WITH EACH SHOT!" "AP Rambo" "She just spams E now" "3/4 of her kit are shit" "She is thematically inconsistent" "She zeroes in and focuses on a single target to duel them" "She is no longer a poison mage" "She's too much of a hyper-carry" "She fights with an AK-47" etc.

All of the above are criticisms of Cassiopeia, and to varying extent they are all true. But overall they all tie back down to just one simple fact. Your rework made her too brutish.

Does that mean she is a brawny tank like Sion? No. Everything is relative, but when you take a cowardly laid back noblewoman and give her an AK-47 to use as her primary method of attack, that's...wrong. She's a Poison Mage so her primary source of power should be her poison.

It SHOULDN'T be impactful and sudden like your typical nuker. She should be gradual, starting off weak but slowly growing deadlier after a while until the enemy has no choice but to GTFO before the poison becomes even more severe. There are hundreds of ways you could have achieved this.

Have the poison start off shitty, but gradually ramp up into something fierce, have the poison gradually eat away at the enemy's stats until they have the power equivalent of a champion several levels below what they actually are. Don't give me that "DoT is un-appreciated". It rang hollow when you followed it up with "she stepped on Brand's toes" and it rings hollow now.

Anyone can be balanced, anyone can be viable. That's the easy part. What you need to do with a rework is to create something that is FUN first. Something that fits the champion. Balancing can be done once you strike gold since it's a simple matter of throwing numbers in till the masses like it. You may disagree with this philosophy, but this is exactly what happened with 4.21 anyways. You threw in enough numbers until the masses started warming up to her gigantic set of numbers.

This rework is the LoL equivalent of taking Captain Janeway and turning her into a Salamander.