I understand that he can endlessly stacks health so he need a bit lower health to compensate, but his base health is even lower than ADCs.
For comparison : Sion 543 – 1784 Ashe 528 – 1871 Vayne 498 – 1909 Amumu 613 – 2041 Sejuani 600 – 2215 Alistar 613 – 2415
He need to farm a lot just to be even with other tanks. (Look at Alistar's 2.4k health >.>) It almost feels like his health stacking is his weak point.
Sion has 19 more base health than Mordekaiser.
Hell, this is some next level stuff
its pretty much implied you get between 400~1200 free health depending on the length of the game, farm and kill participation.
i normally reach 6k hp on sion full build. i dont hit 6k on any other champ. hp isn't my concern.
He need to farm a lot just to be even with other tanks.
Your definition of "farming a lot" is, how do I say... very low?
You need 200 CS, without any single kill/assist/large monster, to have as many HP as other typical tanks. If you have 5 monsters and 10 kills/assists (and it's easy to get assists with Sion), you barely need 130 CS. Sion's baseHP+passive is typically higher than anyone baseHP, except maybe Alistar, unless you've been severely shut down in early game.
The bonus health Son gets from his passive is also multiplied by Cinderhulk. When you consider he hasn't spent any gold on this health, it's really more like 'base health', but still treated as a bonus for being multiplied.
Meanwhile 
In order to brake even with amumu(@18), one needs merely ~100cs, probably less if you get the cannon creeps and maybe a kill or asist.
Given how potent his kit is in lane, even not that good players should reach that at ~min 15, so around lvl 12already.
For Sejuani, thats another <100 cs, which should easily be possible to reach befor lvl 18.
Actually, i usually find my self with 500hp from W at 20min, which meanslvl 14-15.
at that level, your even with alistar, who has the highest basehealth in the game, and unless you stop getting stacks, you will outscale him from there.
Also, don't forget he has a maxhealth scaleing shield.
If you compare him to nauti(576 – 2038hp),who also has a maxhealth shield, you need just about 100 cs to break even, so most of the time you will be 400+ hp up on him at lvl 18.
I don't necessarily agree with this. While it's true that Sion Does has less Base HP than most tanks, His Passive W is what allows him to get more HP as the game progresses (Assuming you don't suck at farming). While this means he can be a bit squishy early on, He can hypothetically scale hard into the late game. However, Certain champions & Items can prevent him from being a threat. This way he's not TOO OVERPOWERED, but can still be an efficient tank in the right hands.
Item's in Question:
Champions:
If I'm forgetting any items, Champions, or just COMPLETELY wrong about Sion, feel free to leave your two cents.
It has some to do with his passive. Each hit point he has comes back to make his passive longer. His shield strength also scales off of HP. I checked Nautilus. His shield also scales off of his hp. He starts at like 576 and goes to 2038. Still good, but at that point you only need 100 ish CS to make up that extra 300 hp. (10 from cannon minions helps a lot). That is my uneducated guess anyway.
