Let me tell you little bit about what Fizz is like.
Arguably the WORST laning phase in the entirety of league of legends. A champion that from level 1 is zoned off of creeps in atleast 85% of his matchups. Often times taking 1/5 of his HP the moment he decides he wants just a single melee minion. This becomes literal hell when you face off against champions who are all about consistent poke and harass, think Jayce, lissandra, Orianna, ect.
Fizz's only real strategy that he's ever had to deal with this is simply hoping to be stronger than his opponent in an all in. You either win your lane from an early point in order to gain lane priority, you fail and lose lane and just multiply the down right infuriating uncounterable gameplay that you were already experiencing, or you play passively and get harshly outscaled as the enemy gains gold and level advantages from your lack of CS and ability to stay in lane for more than brief periods.
Wanna know what D5 and up is like for this? Consistent counterpicking. It's no longer the cheeky plat mid laners who pick veigar, or Lux, or any champion that is essentially a suicide pick into assassins. No, it's always the champions that ruin your day. This has actually gotten even worst with the current pre season. You got Aery that gives them a boost to their poke, you have nullifying orb which will give them added safety against an all in, and you have stop watch which will give them a free escape from your ultimate towards the mid game.
I actually used to be a very kind hearted person. I had never flamed, never trolled, and never exhibited even a single instance of toxic behavior in a whole 2+ years of playing league, then I got to diamond with Fizz. See up until that point it was easy to abuse players as I noted before. They never did anything properly, they never brought the right summoners, Itemization, and always picked suicidal matchups. Once you get into diamond elo this all changes. The consistency of enemy mid laners doing all of those things right SKYROCKETS. They abuse you. Granted it won't be 100% because it's still diamond, but to compare it you'd get players who screwed themselves atleast 3/5 games, in diamond it was more like 1/5.
This actually caused me to quit league for 2 months because even with 70%+ win rate with high diamond MMR the gameplay was just too damn frustrating to continue with. Then the rework came. The rework proceeded to gut fizz as a champion. Basically the rework took fizz, examined him, and then went through every ability and thought of what sort of counterplay could be added. His Q and E were already pretty heavily dodgable, his R was as well but they nerfed that further alittle later. So they added a delay to his W and penalized using your R in any instance besides ranged initiation. They didn't have to touch the passive because that was already weak, outdated, and made no statistical impact on the game.
They added the mana refund on his W, but even that was given couterplay because it doesn't refund the full cost unlike every other farm focused mechanic in the game.
Now down the road fizz is still horribly abused during laning phase, lost overall power so he's less likely to actually kill his mid lane and gain lane pressure so he can actually touch minions, and his late game is a mess. His R is a slow narrow skill shot that is required to hit because it both equals 1/3 of his primary damage and also is the only way to setup his W without dying trying to proc it. His E is slow and massively dodgeable, and most importantly is his short range Q being dodgeable which is almost going to be his only way of managing to use his W in the first place.
What probably annoys me the most though at this stage is the reactions that Fizz gets as a champion. I'm permanently zoned from CSing starting at level 1, I have one of the worst laning phases in league and I'm constantly down both XP and gold, but the moment you get a kill as Fizz you get FLOODED with comments saying that he's broken and has no counterplay. Doesn't matter what shit you have to put up with when playing this champion, doesn't matter that you're an assassin, doesn't matter that you're putting in effort to play this champion or secure kills, you're just broken, require no skill, and have no counterplay.
Remember tank fizz? This is why it exists/existed in the first place. Pre rework fizz (and post rework to a lesser extent) had stats that were viable enough for a tank build, and by going tank build you could get rid of all of the problems that Fizz had to face due to his kits design.