Before I get started this was originally its own thread but it got deleted because I didn't ask for permission to make it (sorry mods). Below is everything from the original post plus a few other new things.
Disclaimer: I am not a very good pokemon player and I probably don't understand the adv metagame completely so take everything I say with a grain of salt and correct me if I am wrong about anything here. That being said this is my opinion so please respect that this is just how I feel.
Ah yes dugtrio. Personally I feel like it should be banned but that is mostly because if dugtrio is banned it would more than likely open the floodgates for many...MANY UUBL pokemon to become viable. I wish to discuss a few other specific pokemon another time (especially blaziken) but the one I wish to talk about today is raikou. The best and worst part of raikou is it's speed (which is the same as starmie's). It outruns most of the metagame which is super important in a world where choice scarf doesn't exist yet. However (though I am sure there are some other reasons) the main reason raikou is still not in ou is because of dugtrio. It is faster than raikou and unless raikou completely invests in bulk (which isn't ideal and I am pretty sure still doesn't help that much) it gets one shot by the usually choice band boosted earthquake. Obviously this is a problem for many pokemon (and one of the reasons uubl is so large) but this really hurts raikou the most because in my opinion if dugtrio is banned not only could it be a top tier threat...but it may also be ban-worthy itself.
"But why is raikou ban worthy," you may ask. To put it simply it has access to calm mind substitute and out speeds everything in ou except starmie (which speed ties it and also doesn't matter that much) aerodactyl and jolteon. To say that raikou could probably beat most of the metagame with just lum berry as the item calm mind substitute thunderbolt and hp ice isn't exactly a lie. The only 3 pokemon (in ou) that don't get completely destroyed by this set specifically are blissey swampert and aerodactyl. However even these 3 may have trouble. Even blissey is screwed if raikou boosts itself enough (and it doesn't have calm mind itself) swampert is especially screwed if it is hp grass (but hp ice is neutral so this might not even be necessary) and aerodactyl not only cannot switch into a substitute but since earthquake isn't stab raikou could possibly tank it if it wants to do hp or defense investment.
"But what about other ground types," you may ask. To put it simply hp ice destroys each and every one. Neither flygon or claydol are safe to switch in on raikou because (especially when boosted) hp ice beats both of them. Neutral targets don't fare much better as not even fortress is safe from a thunderbolt coming off from a plus 2 special attacking raikou and nothing in ou can resist both thunderbolt and hp ice.
Now I did fail to mention snorlax which has high special defense rest and access to earthquake. However it is not safe from raikou either as if it is forced to use rest it is leaving raikou to spam calm mind freely and even if it has sleep talk it is pure rng whether snorlax uses the right move and snorlax probably doesn't like to run sleep talk anyway because then it needs to get rid of curse which is a big problem. And that is only for curse sets as if snorlax runs an all out attacking set raikou can just substitute if it knows snorlax with self destruct.
Of course raikou can't beat everything by itself as some things come down to chance and it doesn't exactly have high defense. However it can be easily helped by its teammates (immediately gengar comes to mind) so that can be easily offset. The best thing you can do against raikou is use roar if you live an attack which comes with its own problems.
All in all raikou would probably be top tier at the very least in adv ou if dugtrio is banned. We need to watch out for raikou if dugtrio is ever banned as it really could destroy ou with a combination of everything mentioned above. Please don't let this post make you afraid to ban dugtrio if the time ever comes because banning raikou or any other huge threat is always possible.
To add to the original post I did forget to mention magneton. Technically magneton does resist both thunderbolt and hp ice. However this still isn't a valid answer imo because the 2 offensive moves it runs (thunderbolt and hp fire) do not do enough damage to justify being used as an answer. Raikou could just calm mind and then wreak havoc when it is ready.
ADV is not DPP, BW or any future gen after the GBA era. And because something is banned in modern gens should not be retroactively banned. Period.
Arena Trap is perfectly balanced in Gen 3, this is because there aren't pivot moves, and the threats / sweepers you have at your disposal are quite weaker than what you will have in the modern gens. U-turn doesn't exist, Volt Switch doesn't either. So you have to rely on Baton Pass so Dugtrio can pivot in. If you don't have a pivot move, you have to actually double switch a 35/50/50 bulk Pokémon into the field.
The three Pokémon in OU that learn and use BP are Celebi, Zapdos, Umbreon and Jolteon. Umbreon is not used alongside Dug usually, the other three can and when they BP they have to be careful since if they go first and opponent attacks, your Dug won't do jackshit.
Special and Physical boosting is limited to +1 per turn in most of the viable mons (the only exceptions being SD pass Celebi, Heracross and Marowak. Stuff like Blaziken never uses it and tbh out of the 3 Pokémon I mentioned Celebi most of the time does not use it since it can pass other stats too). So, to sums up, both ofensive and defensive options are weaker, therefore removing a piece from a team with Dugtrio does not just ruins the metagame!
And as for Dug itself dueling with its target, you have to first chip it down. EVs placed in specific ways so you live another hit from a mon in the tier is something that happens in EVERY generation, in both OU and VGC. From this meta in gen III, up to gen IV OU where each mon that can will run 219 speed to be faster than Kingambit.
But how well does dug into these targets by itself? Is it that broken so it can idk, delete swampert and allow Metagross (?) to win the game by itself?
Bulky Dug (because, spoilers, you need to tank a Blissey Ice Beam in order to beat it since it doesn't OHKO) vs Swampert
252 Atk 30 IVs Choice Band Dugtrio Earthquake vs. 248 HP / 216+ Def Swampert: 141-166 (34.9 - 41.1%) -- 68.9% chance to 3HKO after Leftovers recovery
0 SpA Swampert Surf vs. 136 HP / 24 SpD 30 IVs Dugtrio: 234-276 (95.5 - 112.6%) -- 75% chance to OHKO
Bulky Dug vs Special Lead TTar
252 Atk 30 IVs Choice Band Dugtrio Earthquake vs. 252 HP / 16 Def Tyranitar: 323-380 (79.9 - 94%) -- guaranteed 2HKO after Leftovers recovery
240+ SpA Tyranitar Ice Beam vs. 136 HP / 24 SpD 30 IVs Dugtrio: 238-280 (97.1 - 114.2%) -- 81.3% chance to OHKO
Bulky Dug vs Zapdos
252 Atk 30 IVs Choice Band Dugtrio Rock Slide vs. 4 HP / 0 Def Zapdos: 204-240 (63.3 - 74.5%) -- guaranteed 2HKO after Leftovers recovery
252 SpA 30 IVs Zapdos Hidden Power Grass vs. 136 HP / 24 SpD 30 IVs Dugtrio: 195-230 (79.5 - 93.8%) -- guaranteed 2HKO
Dead after one Spike
Bulky Dug vs Blissey
252+ SpA Blissey Ice Beam vs. 136 HP / 24 SpD 30 IVs Dugtrio: 207-244 (84.4 - 99.5%) -- guaranteed 2HKO
Dead after one Spike
252 Atk 30 IVs Choice Band Dugtrio Earthquake vs. 0 HP / 252 Def Blissey: 350-412 (53.7 - 63.2%) -- guaranteed 2HKO after Leftovers recovery
I could go on and on with Jirachi, Snorlax, Suicune... And not make me start with stuff like Gengar, Flygon Claydol, or the fact that after locking on Earthquake DD Mence, CB Aero, DD Gyara, Sub Zard, Sub Petaya Hera etcetera.
Let's also use WRs. Using
Jimvitational usage stats from rounds 1-9, Dug has a 20% usage (healthy in a gen with much less Pokémon than a modern one) and only 47~% winrate. Why wouldn't a broken mon be used in every single team or have an insane WR if played "only" 62 times out of 310 games? Simple, it's not broken, it has never been. Dug is a base 80 attack mon, which using it at high level requires a lot of skill actually.