New Katarina is a joke. For one she goes against her primary design philosophy, which is mainly to do with her passive. She's supposed to be the Reset Queen, not the queen of "get one kill then escape". Essentially what I'm trying to say is her passive is no longer a core part of her identity and it's not very useful either on her current kit.
Her reworked Shunpo is just nonsensical with its cast time, making it a clunky and overall slow move. Her reworked W is just dull. Her reworked Q....I could honestly live with IF it was on old Kat. And her ult is the same thing.
Additionally her previous kit flowed well with itself, where this new kit just feels clunky. Trust me, after messing around with both Kats for a few 100 hours in games I can safely tell you that the new Kat is a lot less satisfying to play and a lot more boring.
There was no reason to rework her outside of "she stomps in low elo, let's change that". Good? If a champion is stomping in LoL elo then that means that they'll eventually figure out how to beat her, and thus they'll figure out how to beat each champion systematically by taking advantage of their peak points, and so you will get low elo players learning how to climb.
The argument of "her burst damage is unfair" doesn't really apply since her burst damage was only really effective when she got ahead and any champion can one shot you if they get ahead enough.
"But new Katarina takes more skill to play!", false, I figured out how to play her within the first hour of playing the reworked version on the PBE. She's not hard at all. Ironically enough I'd say that she's easier than old Kat since the whole dagger mechanic explains itself more clearly than old Kat's kit. For one she lacks her ward hop, which was actually one of the things that separated good Kats from great Kats, mainly to do with speed and efficiency. She also lacks any form of strategic combo now outside of E->W->Q and Q->E->W....oh wait, they're exactly the same as old Kat's combos.
"But old Kat's abilities were targeted!", this Kat's abilities are also targeted and her daggers aren't hard to hit, so your point?
"But she's stronger now", stronger takes the back seat to fun and fulfillment. If I'm playing LoL I'm playing it primarily for my personal enjoyment, not to just win. Being stronger means that she has to have a lamer design aimed towards the meta. The previous Kat was great in the fact that she was satisfying to play AND she was never "unviable", she was always borderline meta or tier 2-tier 3, which isn't that bad for people who dedicated time to her.
With this new Kat I don't feel any fulfillment for a play I make. New Kat isn't fast or fluent, which was the primary reason as to why old Kat was so much fun to play.
It seems like Riot just want to tone down the game's speed so that they can fulfill their fantasy of having tanks be unkillable behemoths who out DPS you, ADCs who have infinite mobility, self peel and damage, supports who shield for 5k+ and junglers who are tanky and gank 24/7. Oh wait, that's literally just competitive. Oh right, they don't want fast champs in the game because otherwise people might not play their meta champs which are used in competitive. It's almost like they base their balance predominately off of professional play since that's how they make a lot of money.
As for my overall opinion of new Kat....I don't hate her if I forget that old Kat ever existed....however, since old Kat did I hate the new Kat's guts since this piece of garbage is supposed to fill the place in my heart that old Kat took up. She was my main passion for playing and they took her away from me and many other Kat mains who loved her.