As a non-Cassiopeia main, I'm going to try and sum up the frustration of the Stashu remake

I have never been a Cassiopeia main, but b4 the remake, I played her once in a while

However, since the remake, she is a joke, and heres why

She is: Squishy, immobile, skillshot dependent for her E, E dependent for damage. Her ult is her only defense and skilled players can stop it from affecting them. She has horrible ratios so her early game is terrible, she has to try and stack a passive that "balances out bad ratios" so she can stack AP. She has to spam E for 90% of her damage, but it can't be spammed unless she hits Q/W. Her kit is just a giant mess of constraints and contradictions. She gets blown up in 1/2 a second but has to be in 500 range to do damage. She has mana problems but has to spam in order to, again, stack passive.

In order for Cass to be good, she needs:

A new passive, one where 90% of her gameplay doesn't revolve around it. Damage spread throughout kit. E doing everything is just a terrible model that will cripple the champ. Range and consistency. She is squishy. Trying to get close in teamfights as an immobile squishy mage is a terrible idea. Good ratios. Again, she needs damage on more than just E and needs to be able to build different than just straight glass if she wants to participate in teamfights if she wants to be viable.

At least that just my opinion of whats wrong with her. Stashu will just continue to do w/e b/c his version is best