Why I think Kassadin's rework was ultimately a failure
There are several problems created and several problems that weren't fixed with his rework. I'm not saying that he needed to be an abomination with a 99% banrate, but his newer kit has caused a lot of issues that aren't as apparent.
1) His pre-six is too good now
While his pre-six is really terrible against something like Zed, his pre-six is oppressive against a lot of mages. Riot made him go from a champion that usually needed to work to survive lane against everything to varying degrees to a champion that is either countered or bullies the matchup. His Q shield is completely broken in lane and combined with the high ratio/early base damages on his Q, it is awful to play against as a mage since instead of being unable to cast a spell, you waste mana without actually hurting him.
Continuing on this issue, his W change is a big problem in two ways; it made his CS'ing much easier so there's less of an excuse to miss minions, and the AA reset made it easier to CS under tower. He is getting a lot of compensation to his laning phase for the silence removal/ult changes, and the W by itself is an incredibly broken ability since it's an AA reset that not only does a lot of damage, it also gives you a big chunk of mana. It's just an all-around overlaoded ability.
2) He still does the same stuff he'd do before, he just needs to be a bit more ahead
That might be a bit confusing, but it's about his general attack pattern. If Kassadin would've one-shotted you before the rework, he'll still be able to do basically the same thing. He just requires a bit more gold. His attack pattern of instantly appearing from nowhere and then melting your health bar in the blinnk of an eye still exists. It's just a bit easier to get away if he's not able to completely one-shot you, and his damage is a bit lower. He's just a watered down version of the monster he was before
3) His ult feels more powerful in some situations even if it's completely weaker
The reduced ult CD means that in short fights, he is going to be completely ridiculous looking. He blinks around even faster and fights even faster. Even if he cannot do it for as long and it does less damage, it has the illusion of being way more powerful than it actually is, which is demoralizing to the opponent and makes him annoying to play against. His ult was also buffed in a few niche situations in conjunction with his W, since there's one really troll AD/mana build that you could do to get a 1.8 second CD on his ult and always be guaranteed to have enough mana to ult as long as he had a champion to W. His ult was a lot stronger in this situation. Basically, it had the illusion of being stronger and more mobile even if it was definitely weaker. It isn't too bad anyways, since a good Kassadin can manage his stacks.
4) He is less complex and ultimately less rewarding for understanding his kit
This complaint is one that's a bit more personal and less about how unsatisfying and annoying he is to play against as a bursty mobile death machine. With the rework, the changes to his passive and the many buffs to his pre-six laning phase made the few cases of smartly abusing his passive to win a fight disappear entirely. He was made much more skill-less since his pre-six went from being defined more about survival to being defined about how quickly you could win a war of attrition by abusing Flask/Q trading power. I can't really speak too much for his pre-six abuse cases pre-rework since I was bronze and played on 200 ping for most of his time before the rework, but right now, it is hilariously easy to kill most mages in lane as Kassadin. His Q shield added a lot of durability and room for mistake while his W change and passive change made it less necessary to commit for a chance of winning a duel. Riot pruned strategic options from him in favor of just making him more reliable to play in a very binary manner.
tl;dr kassadin lost most of his depth, still feels oppressive to play against, has too easy of a pre-six, and all-around has a lower skill floor/cap.