Hi Sunday Cake here, Gold 3 this season.
I main Sion since season 4, before and after the rework update.
And i can say his rework was a successful one all things considered , he's more "Fun" to play and feels more interactive than ever.
those things said i think he's on a bad spot balance wise currently. ( Patch 6.13)
the champion himself has been unchanged since the season 5 end but a lot of his tools were completely destroyed
To state some of them :
-Grasp of the Undying nerfed, Cinderhulk,Sunfire,Gauntlet,Frozen heart
and sure he's had a lot of cool stuff like tabi's being buffed and spirit visage buff on health regeneration ect.
but Sion's base stats are weak to begin with,he is not tanky AT ALL early , he's got less base hp than Tristana and half of the adcs, and his scaling hp per level is just AWFUL.
- "Hey but he has hp scaling that can scale infinitely with his W !!"
The maximum you'd get from the W passive if you're doing a REALLY good game is around 1000 bonus health and if you're not doing good you'd get 300 to 400. you'll have lower health than everyone else with the so called infinite hp scaling tank. (for example someone like Alistar or Darius/ Udyr hp scaling will get 600 health more than you just from leveling up , doesn't need to farm minions to get that value)
-"Hey but his base damage is good, that must compensate"
I admit his Q damage is insane early and his base ad is okay too. but you must keep in mind that damage isn't as reliable , All of his skills are slow predictable abilities. the only skill that he can chain with Flash is his W ( that's why i play with smite top still :D ) .
Don't get me wrong , it's not a bad thing that he is predictable, that's healthier gameplay and it feels like that's how he's supposed to be given his huge body and menacing look.
What is bad is the combo : Not so reliable damage + Not Tanky
The only times i felt tanky was when i was extremely ahead or when my team was .
I'd like to see Sion with more health scaling with levels or with his W passive. sometime I feel like i'm playing ADC when i get popped in 2 3 seconds in teamfights, a tank with the scaling hp gimmick should be more of a sturdy wall than he is now.