Well that was unexpected!
We are used to cards being banned at the usual scheduled banned and restricted announcement times, but Wizards of the Coast has decided to move on Nexus of Fate in Arena best of one outside of that schedule because it is not an official paper format. This has been a hot topic issue in Magic for a while now, with Nexus of Fate proving frustrating but not overpowered in normal Standard and a real hassle in best of one due to its polarizing nature.
There are a multitude of factors at work here to discuss, but we will start with the card itself.
Nexus of Fate is an Egregiously Designed Card
I have no problem saying that the fact that Nexus of Fate was even printed in the first place is astounding. Its design is so bad in such painstakingly obvious ways that the fact it ever saw the light of day is confounding.
This is not a power level issue, but an issue of designing fun and healthy Magic cards.
Nexus of Fate is going to do one of two things:
1. It is going to take most to all of the turns in a game.
This, of course, is the result we often see in Standard. Almost all modern Time Warp effects are printed with the "exile this spell upon resolution" template we see on Karn's Temporal Sundering, Part the Waterveil, Temporal Tresspass, and Temporal Mastery. This is not complicated, as it's clear that allowing a player to take many turns in a row just isn't very fun from a game play perspective.
Typically the idea is to keep it out of the graveyard, as that is the easiest spot to recur it from, but it feels like Wizards thought they were being clever by "hiding" it in the player's library instead. The problem is that this gives you an infinite amount of Nexus of Fate to cast over the course of a game, and when combined with powerful tool s like Search for Azcanta and Wilderness Reclamation this becomes a very real possibility.
2. It will rot in trade binders because a 7 mana Time Warp isn't playable.
If tools like Search for Azcanta don't exist and taking all the turns in a game isn't a viable strategy, then Nexus of Fate is likely to rot away in player's binders because paying seven mana for a Time Warp isn't a desirable effect in Standard. Time Warp effects require very specific game states to be worth playing, usually only seeing play in decks that overload on them and plan on taking many turns in a row. Even at five mana Time Warp would not likely see play in Standard on rate alone, making Nexus of Fate either extremely fringe or unplayable.
So we've got to possible outcomes for a card - it exists to take all of the turns and not allow one player to play the game, or it exists to do nothing.
That's bad design, as both outcomes aren't very fun; there's no middle ground.
(And of course all of this is ignoring the stupidity of a tournament-playable card only existing in foil and not in booster packs. I never thought I'd see the day when we saw a Mountain with sharpie of a card name written on it on Pro Tour coverage, but Nexus of Fate saw it done.)
They can't go back and un-print the card, despite it being a flawed design, so the only real choice is whether or not to ban it.
Banning Nexus of Fate in Best of One
Realistically Nexus of Fate is not an overpowered Magic card.
Sure, it can create powerful game states when used in conjunction with other cards and be a game winning strategy, but it's certainly nothing compared to something like Skullclamp or even Saheeli Rai/Felidar Guardian. Any sort of pressure backed up with disruption like Negate, Duress, or enchantment removal for Search for Azcanta or Wilderness Reclamation will usually be enough to take the deck down, which is why the deck only sees moderate success in tournaments.
The problem of course, is that these cards are sideboard cards.
Sideboards don't exist in the popular "best of one" Standard format on MTG Arena.
This has an incredibly polarizing effect on the format. Decks that exist on the extreme edges of the metagame, most notable Mono-Red Aggro and various creatureless Nexus of Fate decks, prey on the fact that nobody can bring in a few haymaker sideboard cards and defeat them easily. Furthermore, these decks exploit the fact that players are stuck with their ineffective maindeck cards if they are trying to be reactionary - think about those worthless Lava Coils against Nexus of Fate decks and that laughable Thief of Sanity against Mono-Red Aggro. By presenting a very linear plan of burning your opponent out or taking infinite turns, you don't need to bog yourself down with reactive cards that may or may not be good.
Neither Mono-Red Aggro nor Nexus of Fate decks have been dominant (or even that good) in best of three tournament play, but are obnoxiously omnipresent in best of one play for these reasons.
Banning Nexus of Fate in best of one is putting a band-aid on the format's primary problem - it encourages you to play a linear deck that is hard to interact with, and as long as there is something linear and powerful to do people will do it.
Still, if Wizards of the Coast wants to encourage best of one as a format (which I think is a good idea as it's a great option for Arena) then being more active with banning cards is a way to help regulate the format. It's not pretty, but it works.
Banning Nexus of Fate in Best of Three
So that begs the real question: Should Nexus of Fate be legal at all?
While the power level of Krark-Clan Ironworks was definitely higher than Nexus of Fate in Standard, it was not an overly powerful or oppressive deck. It did well at high level events, but was not a dominating force when it came to raw numbers of people playing the deck. There were good sideboard cards against it, and it was certainly beatable.
A lot of the complaints about Krark-Clan Ironworks were based around the things it did to a game of Magic. Aside from being completely miserable to play against, it also abused small rules interactions that were not intuitive, like the overpaying of mana for simultaneous sacrifice triggers. Furthermore, it contained a number of loops that were difficult to properly execute on Magic Online and hard for newer players to understand. The deck wasn't an overall positive for the format and even the players who played it and did well with it agreed the format would be better off without it.
Nexus of Fate has similar issues. On MTG Arena there is no chess clock, leading to game states where a player is just continuously looping Nexus of Fate as their only card in deck with no way to win and both players are just sitting there hitting okay. This is of course partly an issue with the MTG Arena game clock system, but happens in paper too.
Player1 has no library or hand, and is using Teferi to "tuck" itself every turn. Player2 has no permanents in play, 1 card in library, 7 cards in hand (some are lands), and is discarding Nexus of Fate every turn. What happens? #MTG pic.twitter.com/5DPUztU3nO— Brad Nelson (@fffreakmtg) February 8, 2019
These sorts of things just aren't healthy for the game.
Nexus of Fate just isn't a fun or healthy Magic card. From the effect it has on games and tournaments to its physical existence as a foil-only card it is a miss on almost every front.
Now I understand that banning a card from Standard is a very bad thing to have to do.
We've seen numerous Standard formats in the last few years ruined by poor design and banned cards, but this is more than just a power level issue. Banning a card for power level reasons feels awful, because there's always going to be "best" cards in a format and it creates a cascading effect of people just screaming to ban the next "best" card. But this is more than that.
Ban Nexus of Fate Completely
Banning Nexus of Fate in MTG Arena best of one is a reasonable if awkward thing to do, but it would be better if we just went all the way.
Having a separate ban list for MTG Arena best of one and best of three, as well as paper tournaments is an unnecessarily awkward and confusing situation. It creates a disconnect between the paper and online game, as well as a disconnect between the two major formats on MTG Arena. The more casual players who play best of one occasionally are going to tune into next week's Mythic Championship and wonder why Matt Nass is casting Nexus of Fate over and over again when the card isn't even legal on MTG Arena. They're also likely to just turn off the broadcast and not bother watching again.
Nexus of Fate should not be legal in any Standard format.