I'd say the fundamental issue with Arbok is that it both warps, or should I say, wraps, the meta while having its main counterplay being RNG.
Everyone has talked about this before, but we have very little that outspeeds Arbok. While Pidgeot would probably be used on every team regardless, players are forced to put Primeape, Ponyta, and sometimes even Abra just to have a consistent Arbok mu. And while these Pokemon don't have terrible Arbok mus, they aren't great for Pokemon meant to actually deal with Arbok. Pidgeot can't OHKO Arbok without a crit Hyper Beam, while Primeape is forced to choose between doing 43-50% with Hyper Beam or do no damage with Body Slam to pray for paralysis. Ponyta, meanwhile, only 3HKOes Arbok with Fire Blast and relies on burns to actually deal with it. Thus, Arbok has high odds to tank whatever is thrown at it and just Glare and Wrap its foe to death, or even 2HKO Ponyta. Arbok generally shouldn't face these foes, but even if it does it has high odds to outright win against its "checks". And these Pokemon are exploitable too. Pidgeot and especially Primeape tend to lose or trade with physically bulky Pokemon like Sandslash and Weezing, and the latter especially has a terrible Parasect mu. Their use early-game is often limited, while Arbok tends to have no such problems since it just Wraps out of terrible matchups. Ponyta meanwhile thuds into Flareon and the rarer Tentacool, and has to rely on its own partial trapping move to escape. Arbok has an easy time escaping these Pokemon, especially since they'll often switch in while using Wrap, so Arbok can often get its teammate in safely. The opposing player can double, but these tend to be predictable, and Arbok can often Wrap for a few turns before committing to a switch.
Then we get to probably the best answer to Arbok, Abra. It can't even OHKO Arbok without a crit Psychic, but it can paralyze Arbok and essentially ruin it. But brings up another issue. While it isn't terrible for a great Pokemon to make otherwise unviable ones usable, like how Pidgeot probably single-handedly makes Rock-types worth considering, Abra cannot switch into Arbok at all. This is our best answer to Arbok, something that generally trades one for one, cannot switch in, and is terrible otherwise. While there are some more Pokemon that outspeed Arbok, like Poliwag, Pikachu, and Voltorb, the former is very fringe and not very good, and the latter two are totally unviable. While the duo can paralyze Arbok, if we end up relying on them to handle Arbok then it's proof it's way too much.
Now feeding into my next point, we get into the next way we handle Arbok, praying. Wrap only has ~85% accuracy, and Glare ~75%, which is very inconsistent. Everyone who's played enough RBY ZU has games where Arbok missed twice and died. But everyone has also had a game where Arbok went beast mode and wobbled the opposing team to death. Arbok is consistent enough to where you can rely on it to win games, and when it works, it works way too well. It isn't optimal for Arbok to try Wrapping a Sandslash from full to death, but there are plenty of gamestates where it is Arbok's best option, and both players are forced to bet everything on RNG. And there have been plenty of games where Arbok beat Sandslash one-on-one, and often went on to do even more.
There's also Arbok's pretty good movepool beyond Glare and Wrap. Hyper Beam, Earthquake, and Rock Slide make it quite threatening even without Wrap, and their use as finishers make Arbok getting its foes into KO range extremely easy. If people start using Rock-types to beat Wrap, it'll just start slotting in Mega Drain.
There is the option of PP stalling Wrap, but with Wrap's 32 PP this is very hard to do. While Arbok spreads damage it can switch out at any time, so opponents are forced to reveal their team early-game or risk being predictable, and late-game the Arbok still has a good chance to read a switch for game. From my experience, PP stalling Wrap tends to only work in very specific end-games.
Overall, Arbok is just too good and needs to be banned. It can fail thanks to RNG, but having odds to 1v1 every Pokemon slower than it, and reasonable odds to beat those faster than it, makes it way too much for ZU.
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