This is more of an explanation than an argument, but the "ban those stones" request isn't (or shouldn't) be based on brokeness, since they're not. They're loopholes and inconsistent with the rules. Ultra Bursting Dusk Mane/Dawn Wings are excluded since neither Ultra Necrozma nor Neuroforce are banned, so there's no loophole. But Red Orb, Mawilite, Medichamite, Kangaskhanite, and, in the past, Gengarite were all requested to be banned since they did let you bypass the rules and use Pokemon or abilities that are banned and inherently against the rules. The argument "they're not broken as a loophole so they're fine" doesn't hold water since we're not allowed to use banned stuff on non-broken Pokemon as is, such as Shadow Tag Gothitelle or Huge Power Azumarill. Neither would be broken since those Pokemon suck pretty hard in a BH environment, but they can't access their abilities that make them usable in standard.
Or basically, the point is: "These things are banned, even if you want to use them on things that naturally have them. Oh, but they're perfectly legal if you use this item to bypass the rules on these specific Pokemon. Which is inconsistent with the fact that they're supposed to be outright banned." Alternatively, imagine if standard was like "Drought is banned. You cannot use it, ever. Unless you use Mega Charizard Y. That's okay. But otherwise no Drought ever!"
Again, this is just an explanation, not an argument. I'm not pursuing that suspect/ban since Flint nor Verbatim ever showed interest, that I'm aware of, in closing the loophole since I first brought it up back early in X/Y. So, feel free to agree/disagree with that line of thought. Just thought I'd clarify it since you brought up Necrozma.
Rules that way would be needlessly long. Even if we allowed stuff that would be AG legal it doesn't mean it would necessarily usable in BH. An hypothetical legal Huge Power Azumarill would still have no place in BH. But it is far easier to code "
no Huge Power" than "
no Huge Power except on Azumarill, Marill, Azurill, Bunnelby, Diggersby and Mega-Mawile". Why should the BH rules coder be worried about Mega-Medicham? It's not breaking the game.
And let's not use the "fix loopholes at any costs 'cause inconsistency" mentality, as we can derail into ridiculousness pretty fast. Such as all the following.
We should restrict abilities all mega evolution may gets, because beware
running Mold Breaker twice and Ampharosite Ampharos, that's a loophole. Same goes for changing forms pokemons. And well you can't run Red Orb Groudon, because you can run 6 of them, and that is 4 Desolate Land over the limit. Poor Kyogre faces the same fate. Want to limit them to
*not six*? Well, say hello to the newborn clause.
Either Imposter/Transform is banned or we effectively lower the ability clause to 1, because copying the opponent ability is a loophole to get an ability more than twice. Oh, and because of that and the first, all of the abilities of the megas must be used one less time, because (example) legit mbounce + bh magic bounce + (possible) imposter is 3 times! That means they have to be banned, because 1-1 is 0. Not to mention we'd lose a lot of the good abilities (-ate, prankster, magic bounce) that way. Raise the ability clause to (actual) 3? 3 PH, 3 Regen teams will be extremely hard to take down. All the tricks to change abilities needs to go.
We can break the Comaphaze with Sketch, you need to copy the opponent using Sleep Talk as a Comatose Pokemon but it's techinically doable.
C'mon how is that thing relevant? It isn't, but we are fixing loopholes, aren't we?
Sand Attack, Smoke Screen, Kinesis, Flash, Detect, Camouflage, Lucky Chant, Magnet Rise needs to go, as their Z-move effects give +1 evasion. All the moves that lower evasion must be banned, because Contrary turns them into boosts. That would remove Defog (with no mbounce, -ate spin and defog, hazards are pretty much going to stay in). Acupressure violates Evasion, it must be banned.
We can actually break all the moves bans with Own Tempo Articuno running Fling, Recycle, Mimic/Copycat, Sketch with Leppa Berry vs anything with Damp, Purify, Heal Pulse, Trick-or-Treat (
or level 99 without Articuno tbh) and Metronome; all you need is enough time and OHKO moves/chatter/evasion boosters will eventually show up. Also, when you manage to Sketch a OHKO move, all the team gets to use it with Assist.
So, in 30 minutes of brainstorming I broke 2 bans (OHKO and chatter) and violated 3 clauses (ability, comaphaze and evasion). I don't even know if this was everything possible.
But c'mon ElMustacho, what you produced up there is ridiculous. There must be some kind of line where the meta works well and the rules aren't loophole-proof yet fine. Well we have that line, and is exactly here; even if it is technically possible to pull off OHKO moves, nobody cares; even if Huge Power Azumarill ends up banned, it's not like you can break the meta with it being allowed, nor it would be relevant.
Edit: autocorrect sucks.