It's not that he is underpowered or anything, it is just right now he is in a confusing spot. When Sion's rework first came out, he was unique but with a balanced price. A lane bully tank with a great late game if you knew how to use him, however his abilities could be avoided. Then reworks were made that copied off of Sion, and Sion got nerfed directly and indirectly. Is Sion bad? No, especially if you know how to use him. However now he's the tank (or juggernaut, or etc.?) version of Azir, only a Sion main can use him.
As for many of Sion's problems follow: Problem 1: What is Sion? Is he a tank? A juggernaut? A lane bully? A giga-carry? What is he? He feels like all of these, and none of these at the same time. Well he is tanky with cc, but that cc is unreliable unlike lets say
who you can very much rely on their cc. Now pointing out those two champions, they have gap closure flash style of some kind. Sion does not, not even his ultimate, his ultimate is more ghost with steroids. So that makes him a juggernaut! Well not exactly his damage isn't nearly as high as the other juggernauts. His title even makes him out as a juggernaut but he isn't! Now is he a lane bully? Somewhat a full Sion combo makes dueling him early game a pain, but that combo can easily be countered. Okay so he has infinite HP stack so that makes him a giga-carry! Well nope not really, HP% damage is still so common that a mosquito has it. So his infinite HP hurts him more as the game progresses. So in the end what is he? He really has no defined role.
Problem 2: Too many really bad counters. You've pretty much already lost the game if you pick a Sion or the enemy team counter picks in champ select with
and maybe more. Other tanks can still be useful against these champions since their CC is again reliable. But since Sion's CC is not reliable he can't really do anything against them. Counter picking and hard counter picking is healthy, brings strategy, counter picks that remove champions however is not.
Problem 3: The infamous problem of "why pick him?" Before when you picked Sion his power was unreliable BUT DANGEROUS. Got hit by a Sion combo? OUCH! But they nerfed his numbers so that now he tickles you with his combo. Sion was the only balanced numbers champion there ever was. EXCEPT FOR HIS E, it needed that mana nerf badly, the damage nerf however makes it a useless move now. Anyway you can side step his moves, you can cc him mid Q, you can read what he was about to do, you have a chance to shut down his power. Not like
who you either out number him or he out numbers you. Sion may out number you but you can beat him without having higher numbers. So now that Sion has so much unreliability and no numbers backing it up, why pick him? Top lane? Might as well pick
they have more reliability and power. Jungle?
maybe more, not sure of jungle now in preseason 6.
Reasons to pick Sion: Well if you have fun with him then play him! I still have fun with the big guy and still play him. Also one advantage Sion brings to the table is that his CC may be unreliable but it really messes with the enemies mind. Sion is the master zoner, you don't want to get knocked up by his q, or when he ults you don't want to get hit by that. A real psychological brain busting champion.