Why do people still think that Sion is weak?
He brings as much cc as Maokai, Gragas or J4 does. Like literally. All 4 champions have 3 individual ways to cc enemy champion(s) and peel for their carries. All 3 are powerful initiators who can withstand the initial burst of a teamfight and continue to soak up damage on top of being a threat by proximity to the enemy carries. All 3 thrive in the chaos of a messy close quarters all-in teamfight. Perhaps you play assassins who tend to get both cc and damage, but Sion isn't meant to be an assassin. He looks like a tank. He brings the cc and presence of a tank. He is a tank!
I see so many players try to play him like Wukong or Irelia or Rumble. That's not the way he was built. He isn't an assassin, he isn't a big duellist, he isn't some splitpushing offcarry, he is a cc-initiator-peeler-teamfightin' tank.
His Q isn't meant to be fully charged every time you shoot it. Don't be greedy. You can release it early to slow enemies. You can slow them with your e and then get a guaranteed knockup or force their escape, or more commonly, chain it with your allies cc during a teamfight. It isn't meant to be a reliable nuke to their health bars. It's aoe cc. It's a soft taunt that draws in enemy stuns and other cc abilities the same way a Nunu or Fiddlesticks ultimate causes your opponent to be forced to stun you instead of your friendly Kog'maw.
His passive shouldn't be a guaranteed kill just because you died. The only times I've seen Sion die and get kited through his passive and end up doing nothing with it was when that Sion player got caught out of position without his team or when he died alone as the last remaining survivor after a teamfight. Which if you are the sole survivor of your team as Sion, you didn't do the job of being the teams tank at all. The whole point of it is that if you happen to die during the teamfight (which as the tank, means you are doing your job, buying time for your team to kill them, and absorbing important high impact-high CD abilities), you can shred their frontline and still be a threat.