But the unhealthy burst,
I want to make something clear. The problem is not that there was burst. That is fine. No one is complaining about how Sion had burst. The problem is reliability. When you combine burst with extremely high reliability, you get a champion that becomes impossible to play against whenever they are strong, which necessitates keeping the champion weak.
stun then that meteor thing and anyone who isn't full MR is dead.
Look at Veigar's reliability compared to Sion's. The stun can be dodged quite easily, and the meteor has a delay that allows you to avoid it if you aren't stunned. Sion's Q was not dodgable. His W could be stopped only if you broke it before he could pop it, which was extremely unreasonable early in the game when the values were very high and he could stun you. That's the problem. You can play against Veigar. You can take a large number of actions to work against what he is trying to do. This was much, much less true for Sion.
Kat, goes in and automatic penta.
Again, Katarina is lacking old Sion's reliability. Katarina's ultimate takes a significant amount of time to get the full power off, and that gives you a ton of time to disrupt her and leave her with her ass handing out. Sion had nothing like that. For most of the game, you didn't have an opportunity to respond to Sion's burst combo. He runs up, stuns you, and bursts you with W. There is nothing to miss, no way to stop it outside of blowing up his shield or staying out of his range. That's the difference between Sion and Katarina.
But, there is another component to this. As the game went on, Sion started to become unable to respond to his enemies. He would turn on his shield in a teamfight, it would die instantly, and he would have no options to stop that from happening. That is bad too. It wasn't good that Sion didn't give his opponents options, and it was bad when he didn't have options against his opponent.
Old Sion had a low amount of potential power but had a near-complete guarantee on the power he had. If he was numerically stronger than you, you would lose almost every fight against him. If he wasn't, you won almost every fight against him. That is extremely unhealthy. There was a strong argument to be made for old Sion being the most unhealthy champion in the game at the time. New Sion has a very high potential power, but with lower reliability. He can do a ton of work if played well, but he actually has a potential to screw up mechanically. That is much healthier. It does not appeal to old Sion players because reliability was a large part of what they liked about him, and that is unfortunate.