Sion was one of those champs who got rediculous buffs. Now he's not ass anymore, but his items still are. But since you bring him up, I'll use him as an example.
Tl;Dr at bottom.
Think about this:
before the buffs, sion would get around 400 bonus hp at 20 minutes, and would generally reach 800 hp by 40 minutes leaving him with about 2500 hp without items.
Now, sion can easily break 600 hp by 20 minutes and almost 1000 by 30 minutes. This gives him 2700 hp without items, 200 more hp, 10 minutes EARLIER.
He is likely to have 1200+ hp at 40 minutes, and all this hp just makes it easier to enter lane and into fights which will give him even MORE hp. Thus, he can possibly have 3000 hp without items by 40 minutes. That's INSANE. That buff is absolutely mind bogglingly MASSIVE, and it just managed to put him back to his power level BEFORE the tank rework.
Now if there was ANY better example I would use it, but this is honestly the best way to show how absolutely awful the tank update really was.
That is also why I couldn't play any tank since the update. It was actually more painful than having a migraine (like I thought that was possible). I only play league with my friends now because it has become so unfun that I need someone to cheer me up while I try to play. I won't go on any more because now I'm just ranting.
Tl;Dr The massive sion buffs are a very clear indication that the tank update and all of its items suck.