What's wrong with tank Sion?
I recently played a very low tier game because new season. I as Sion was at some part of the laning phase 60 cs and 3 kills ahead of my opposing top laner Garen. The items that I had were: Sunfire Cape, Ninja Tabi and Null-magic mantle, Spectre's Cowl because their jungle was AP, but she wasn't fed at all, 0 kills. All that Garen had was Phage, Stinger, boots of speed...
Now I would think that with such a huge gold lead and also as a stacking champion with such a good cs count (750 bonus HP)....I would be pretty much unbeatable while facing at least 2 champions. Well it turned out that Garen was still able to trade with me 1v1 even though he always lost at the end cause I use my abilities once he leaves, and when the jungler came I stood no chance...I couldn't beat neither of them because my damage is low and I couldn't sustain their damage for long.
Sion is a very simple champion. This is not good from the users end. Instead it's good from the opponents end because it's very easy to counter you. At the end against most champions, Sion's biggest damage outcome comes from his auto attacks because of how easy it is to counter q and w (largest damage spells).
When building tank you have to adapt to the enemy team comp and not only to your laner. If I as Sion was only focused on bruiser items I wouldn't even have to worry about early adapting, lets say I bought: Titanic Hydra and Sterak's Gage, it would boost up my damage output from Auto attacks, actually make me able to 2v1 easily if ahead, increase regeneration which is useful when laning, and give me a passive that triggers a shield equal to 75% of my bonus health when I'm in danger and this all scales with my HP....
So what's the point of tank items if I cannot even sustain damage from 2 under powered champions?