Nucleon is by far the best Pokemon in Uranium and should be banned from any competitive format - Urayne is not even as close to as good as this thing is, as it doesn't have any spammable strong Nuclear STAB, physical or special. Nucleon is beyond anything we've ever seen - way more influential than Mega Rayquaza, the most powerful actual Pokemon.
First of all, there's almost no reason to not run Choice Scarf with Hyper Voice, Quick Attack, Shadow Ball, and Trick. As tarutaru pointed out, almost everything dies in one hit and only a couple Pokemon in the entire game can even switch into Nucleon. Choice Scarf allows you to get free kills so much more easily, as Nucleon isn't terribly fast, but the point is that often times, you send Nucleon in versus almost anything and something dies. A Pokemon with this capability is far too powerful for any competitive metagame, and solidifies Nucleon as undoubtedly the most broken, metagame defining Pokemon.
Just because you can switch into something doesn't mean something isn't broken. Assault Vest Ludicolo switches into and beats Kyogre, but Kyogre is still obviously broken. Sure, there are things like Soundproof Paraboom and Geigeroach that take no damage from Hyper Voice, but think about what's happening. Nucleon is such a centralizing threat that teams are forced to run the worse ability on Paraboom just to deal with it and/or teams are forced to run Geigeroach, an honestly mediocre Pokemon, just to deal with Nucleon. There are a couple other things that can switch in, like Mega Metalynx, but it's still taking a heavy hit and then Nucleon just switches out.
Again, you can list a couple of things that can switch into Nucleon, but the point is that Nucleon is such a dominant force that you, as the player, are forced to use all of these mediocre Pokemon just to deal with it. And they don't even beat it because the Nucleon player just switches out. Even if you avoid the 2HKO this time, you've taken the hit, Nucleon switches out, and then next time it comes in you're weakened and can no longer switch into it.
The only surefire way of dealing with Nucleon is to have it so at least 4 (preferably 5 or 6) of your 6 Pokemon can outspeed/outprioritize and OHKO Nucleon. That way you don't have to switch into it, because you can't without making your team terrible by using those mediocre mons (and the Nucleon can just predict and deal a heavy hit to your "counter" anyway). So the only team you can use that doesn't give Nucleon a free kill every time it comes in is a team with a bunch of priority users and/or faster Choice Scarfers. Sure, a team like this would beat Nucleon but again, think about what's happened. Nucleon is such a stupidly broken Pokemon that you're forced to use this one very specific archetype of team in order to deal with it. This is not healthy for a competitive metagame because Nucleon eliminates the possibility of any real diversity - teams that either do not follow a very specific archetype or teams that do not carry around multiple mediocre Pokemon that are used specifically to handle Nucleon instantly lose. And no, switching in another Nuclear Pokemon is not an answer, because what if the Nucleon predicts that and uses Shadow Ball? Your method of dealing with Nucleon only works if you guess correctly, and even then the Nucleon can just switch out and come back in later.
The only other option is to Pursuit trap it, but that also causes 50/50s. Let's say you have Pursuit Harptera vs Nucleon. You can kill it with Acrobatics thanks to Gale Wings, but you can also predict it to switch out and kill it with Pursuit. However, this is a 50/50. What if the Nucleon predicts you to use Pursuit and stays in? It then outspeeds you because it's Scarfed and you died without doing any damage to it. You could try running a faster Scarfer with Pursuit, but there doesn't exist a Pokemon which is faster than Nucleon and OHKOs it with Pursuit even if Nucleon stays in. Because of this, it's still the same 50/50. If Nucleon stays in, you hit it with Pursuit, it lives (even though it's super effective, it's only 40 base power and Nucleon isn't that frail), and then you die.
As you can see, there are only 2 ways to win vs Nucleon. Build an insanely offensive team that must have several of the same Pokemon in it that can outprioritize/outspeed and kill Nucleon, use a Pokemon with Pursuit that can normally kill Nucleon and win a 50/50, or use multiple mediocre Pokemon whose sole purposes are to deal with Nucleon and win a bunch of 50/50s in order to prevent your opponent from removing Stealth Rock. Nucleon is beyond any Pokemon we have ever seen, and no healthy competitive singles format can exist with it not banned.
First of all, there's almost no reason to not run Choice Scarf with Hyper Voice, Quick Attack, Shadow Ball, and Trick. As tarutaru pointed out, almost everything dies in one hit and only a couple Pokemon in the entire game can even switch into Nucleon. Choice Scarf allows you to get free kills so much more easily, as Nucleon isn't terribly fast, but the point is that often times, you send Nucleon in versus almost anything and something dies. A Pokemon with this capability is far too powerful for any competitive metagame, and solidifies Nucleon as undoubtedly the most broken, metagame defining Pokemon.
Just because you can switch into something doesn't mean something isn't broken. Assault Vest Ludicolo switches into and beats Kyogre, but Kyogre is still obviously broken. Sure, there are things like Soundproof Paraboom and Geigeroach that take no damage from Hyper Voice, but think about what's happening. Nucleon is such a centralizing threat that teams are forced to run the worse ability on Paraboom just to deal with it and/or teams are forced to run Geigeroach, an honestly mediocre Pokemon, just to deal with Nucleon. There are a couple other things that can switch in, like Mega Metalynx, but it's still taking a heavy hit and then Nucleon just switches out.
Again, you can list a couple of things that can switch into Nucleon, but the point is that Nucleon is such a dominant force that you, as the player, are forced to use all of these mediocre Pokemon just to deal with it. And they don't even beat it because the Nucleon player just switches out. Even if you avoid the 2HKO this time, you've taken the hit, Nucleon switches out, and then next time it comes in you're weakened and can no longer switch into it.
The only surefire way of dealing with Nucleon is to have it so at least 4 (preferably 5 or 6) of your 6 Pokemon can outspeed/outprioritize and OHKO Nucleon. That way you don't have to switch into it, because you can't without making your team terrible by using those mediocre mons (and the Nucleon can just predict and deal a heavy hit to your "counter" anyway). So the only team you can use that doesn't give Nucleon a free kill every time it comes in is a team with a bunch of priority users and/or faster Choice Scarfers. Sure, a team like this would beat Nucleon but again, think about what's happened. Nucleon is such a stupidly broken Pokemon that you're forced to use this one very specific archetype of team in order to deal with it. This is not healthy for a competitive metagame because Nucleon eliminates the possibility of any real diversity - teams that either do not follow a very specific archetype or teams that do not carry around multiple mediocre Pokemon that are used specifically to handle Nucleon instantly lose. And no, switching in another Nuclear Pokemon is not an answer, because what if the Nucleon predicts that and uses Shadow Ball? Your method of dealing with Nucleon only works if you guess correctly, and even then the Nucleon can just switch out and come back in later.
The only other option is to Pursuit trap it, but that also causes 50/50s. Let's say you have Pursuit Harptera vs Nucleon. You can kill it with Acrobatics thanks to Gale Wings, but you can also predict it to switch out and kill it with Pursuit. However, this is a 50/50. What if the Nucleon predicts you to use Pursuit and stays in? It then outspeeds you because it's Scarfed and you died without doing any damage to it. You could try running a faster Scarfer with Pursuit, but there doesn't exist a Pokemon which is faster than Nucleon and OHKOs it with Pursuit even if Nucleon stays in. Because of this, it's still the same 50/50. If Nucleon stays in, you hit it with Pursuit, it lives (even though it's super effective, it's only 40 base power and Nucleon isn't that frail), and then you die.
As you can see, there are only 2 ways to win vs Nucleon. Build an insanely offensive team that must have several of the same Pokemon in it that can outprioritize/outspeed and kill Nucleon, use a Pokemon with Pursuit that can normally kill Nucleon and win a 50/50, or use multiple mediocre Pokemon whose sole purposes are to deal with Nucleon and win a bunch of 50/50s in order to prevent your opponent from removing Stealth Rock. Nucleon is beyond any Pokemon we have ever seen, and no healthy competitive singles format can exist with it not banned.
