''Why isnt he nerfed?'' Simple: his stats arent out of line, his winrate is in a fair spot (50.5% for mid,48-49% for jungle and top), this doesnt warrant straight up nerfs, not even a full gutting. Right now they need to push him away from lanes and towards jungle (which they are doing next patch).
Manaless. Great sustain and waveclear. (Paired with being manaless, he just outsustains every mana champ in mid.) No weak point in the game, with insanely good scalings. Stealth. Spammable resets and the invulnerability that comes with them.
Ugh I hate stuff like this, basically naming everything inside a champs kit to make it seem busted. Oh also it isnt stealth, its camouflage. Here let me do this: Manaless, Free sustain from afking, Good Waveclear, loads of aoe dmg on a single spell, a cleanse for ms, pointn click silence, pointn click true damage execute, Shit ton of free passive resists. Sounds busted right? Yeah I just described Garen. You can make any champ sound busted/overloaded through this.
This champ gets rewarded for THE ENEMY DOING WELL AND HAVING MANY ITEMS.
This is in theory true but in practice this isnt quite right especially this part:
(Even inting isn't that bad if you can manage to kill the turbofed enemy.)
Yes he benefits from possessing fed enemies, so but now imagine this fed enemy dies without Viego on the enemy team, its still pretty bad for your team (also shutdown gold), especially if that fed member is the only with a major lead (if you die at min 50 while 20/0 thats also game losing), Viego simply punishes dying as a fed champ harder. Getting hit by skillshots is bad, getting hit by Blitz Q is even worse, its very similar.
I see him picked or banned in 80+% of my games, since release.
Ah yes exaggerations, Viego never crossed 65% presence, not even right at release, right now he has 40% presence, you also have to keep in mind that when he gets banned by both teams it gets counted extra so his actual p/b is probably lower.
He's simply too good at all points in the game with no actual point of weakness.
No weaknesses? His basekit is pretty underwhelming without his Passive (His W is literally a shitty Vi Q) and having one of the strongest parts of your kit locked behind someone dying is a noticeable weakness imo. All of his Spells are skillshots, you can flash them, dodge them, use mobility spells, his W is very telegraphed, R has a long enough windup. He usually builds very squishy, especially early on so cc and burst him. If hes behind hes insanely useless even if he gets to possess fed members (since youre usually standing in the middle of 4 people and the one person you killed is very likely the enemy adc...) His passive only becomes a problem when behind when his team is ahead enough to kill fed members at which point my point from earlier comes in that he simply punishes dying harder.
Feel free to be frustrated from playing against him, everyone has champs that they dislike playing against something but asking for straight up nerfs when hes not a balance problem is unjustified, he isnt even breaking proplay (yes hes popular but he isnt pick/ban). There are things in his kit that I would personally take a look at (shift power from passive/R resets to Basekit for example) to reduce player frustration, which you perfectly portray here. My main grasp here is that threads like these pop up all the time put they are basically just complaints but dont offer solutions. Some things I'd offer: Stats he gets from Items while possessed reduced by like 20%, Ult damage from reset ults reduced by 20% or so, compensate his basekit accordingly (W would be good). Healing against minions is already getting adressed next patch. You could try that instead of iFrames he simply gets some fat damage reduction (like 90%) and unstoppability this could end up with frustrating moments for the Viego player though (True Damage especially). And no you cant just remove the iFrames since I think nobody would like instant possessions, the delay is good, and no iFrames (or other means of protection) makes him unplayable. I also think just removing him from lanes will already make alot of his frustrating aspects disappear.