The champion isn't useless or unplayable. the champion is fine, your problem is the approach they took to get her here.
The reason everyone is so enraged is that they took away a part of her playstyle with the rework. Cass in lane was a bully because of the damage she could pump out from a safe range with her poisons like Ziggs and his bombs, and Xerath with his mage blasts. The key difference being that she could also use these poisons with E to do insane DPS on a single target, essentially allowing her to chew through tanks easier than most mages in a fashion similar to an ADC. These two playstyles in one champion is what made Cass fun and unique, the long ranged harass and the closer fang shredding. However Stachu's rework removed that harassment capability since 80-90% of her damage is loaded onto her E now, even her poison spells which are meant to serve as a zoning/harass tool in lane do next to no damage unless she Es the target first which puts her in a vulnerable position due to her lack of mobility, and Es shorter range.
The Cass rework stripped her of one of the key aspects of her play style, the poison harass that could chunk targets down until it was safe enough for you to go in and E to death before the enemy burst you first. Now its a build stacks and items until you have crazy enough damage that you can go in and E a target to death before they burst you kind of play style. The key difference being when Cass can do these things, old kit she could do it from level 3-4 to assert her dominance in lane and either get kills or get ahead in lane so she can do well mid-late. Now she can't do that until late mid game- late game when she has her passive fully stacked and a few items to bloat it out
