Canceling Q before the halfway make does nothing useful. And bt the way it is NOT a nuke. Unless you build full damage gear it never gets to be a dangerous amount of damage. Clearly you've never used it.
His gank potential is AWFUL. His ult is bad. Incredibly easy to dodge and lets everyone on the map know you activated it.
He doesnt have "massive aoe", where did that come from?
I can tell you havent played him or you wouldnt be saying these things. Dont just read about a champ and act like you know whats the deal.
All of his AoE skills are some of the largest in the game with his ulti being around the same size as Anivia's ulti and his Q probably being the largest CC in the game unless we count Veigar's Event Horizon and Thresh's The Box and even then those only are only larger because they are a circle all the way around.
His ulti is bad if YOU are bad. Veigar's Even Horizon and Thresh's The Box which I just mentioned are extremely useful skills and are teamfight deciders at points BUT if you are good they are completely useless as you can flash them or even just walk around them and never touch the sides. His Ulti can also be releated to Nocturne's ulti which only hurts 1 champion and has no CC attached to it, Jinx's Ashe's and Ezreal's ultis which are extremely easy to dodge and most of them only doing decent damage if they land it from maximum distance (generally luck) or in Ezreal's case when there are no minions in the way (AKA: Almost never).
What a skill says it does and what it does in practice is completely different. Based on the current meta Karthus should have the most broken, overpowered ulti in the game right beside Soraka having the most useful for your team yet both of these Champions are rarely played when compared to other picks.
Maybe you should get a decent amount of games under your belt with a champion before saying "I Played 1 game with support AP Sion and my Warmog's wasn't boosting my Shields damage, such a bad champ."
EDIT: So at the risk of thinking I might have insulted a challenger player and have that against me I decided to LoLKing you.
You're bronze...and you have 3 games of sion with shit scores and you're build....my god your build. It hurts to look at.
For a decent Sion build you need to look more at a general Darius build. Tiamat will be a starting item followed by some kind of tanky item. You also are pushing for TWO AP Resistance items when the enemy team is more heavily AD, even going after a hexdrinker for Singed when you would have done much better just rushing a brutalizer and dropping him instead of trying to survive the poison.
You not only don't have a right to talk, but you SHOULDN'T BE TALKING. You aren't a good player and you didn't even play Sion right before declaring he was one of the worst champs in the game when places like Elophant and Lolking show an obvious increase in his win rates that put him ahead of several common picks.