This is my personal opinion but I will gladly answer this:
Tapu Koko is currently known as a fast attacker of the team that is ready to handle any variants of Mega Gyarados, and is able to stop FEAR or stall shenanigans using Taunt / Substitute or both. It is able to get past bulky opponents using Charge, and these are main niche of Tapu Koko in current meta. These utilities unfortunately lock Tapu Koko into a very one-dimentional sets, and it is not like it can function as a diverse walbreaker thanks to its horrific coverage outsides Dazzling Gleam, Grass Knot, and maybe HP Ice. Even while carrying these coverage moves in exchange of forfeiting to get aforementioned utility moves, it is not like Tapu Koko is going to beat Dragons / Sturdy's / or Ground-types. This leaves Tapu Koko very predictable compared to Kyurem-B whose sets are clearly more multi-dimentional than Tapu Koko.
It has horrific bulk compared to the rest of the meta and except when maybe using Charge against weak opponent / weak coverage, it will most likely be OHKOed by most neutral Z-moves or any SE STAB moves, leaving it reliant on predictions to block certain moves with Substitute, and know when to Taunt / Charge.
Tapu Koko's offensive stats aren't great, it is Electirc Surge what fuels its power. Its coverages are significantly weaker:
252+ SpA Tapu Koko Hidden Power Ice vs. 0 HP / 0 SpD Landorus-Therian: 280-332 (87.7 - 104%) -- 25% chance to OHKO
252+ SpA Tapu Koko Hidden Power Ice vs. 0 HP / 0 SpD Garchomp: 268-316 (75 - 88.5%) -- guaranteed 2HKO
252+ SpA Tapu Koko Dazzling Gleam vs. 0 HP / 0 SpD Kyurem-Black: 254-300 (64.9 - 76.7%) -- guaranteed 2HKO
And I am more than sure that Landorus-T runs more bulk than just 0 HP. Tapu Koko's STAB is devastatingly powerful and is able to run both physical and special Electrium Z sets, but that's about it. Electric-immune 'mons beat this guy without a question, and those who live Gigavolt Havoc is most likely able to OHKO Tapu Koko in return, and if Tapu Koko is running bulk to compensate this situation, that means it is compromising its power at the same time.
The most-reliable checks:
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and others...
This is another guardian deity that has terrain boost. This time, it is Psychic Terrain that leaves Tapu Lele powerful, and unlike Tapu Koko, it has Fairy-typing as well which lets it have much better matchup with Dark-types. It also has access to utility move such as Taunt to shut down passive threats, Calm Mind to deal with Tapu Fini / specially bulky foes, and can make much better usage of Z-Reflect to get past Mega Gyarados, Golem, and Donphan. These aren't to be considered as the only strength of Tapu Lele, however; Tapu Lele has significantly better offensive stat (130 Sp.Atk) that allows it to use Shattered Psyche to outright OHKO anything that is not bulky enough, limiting its checks even narrower. However, Tapu Lele has the same issue with Tapu Koko; it is predictable. Tapu Lele usually can't afford to run more attack moves other than Psychic / Psyshock and Moonblast unless it is running choice item, because it will have harder time against Sturdy users and stalling opponents. In addition, Tapu Lele does not profit too much from running extra coverage outsides its dual STAB moves because it is not like it will beat Celesteela even if it has Thunderbolt or Magnezone with Focus Blast. Therefore, Tapu Lele's offensive trait in 1v1 is quite predictable, and Steel-types are known to easily keep it in check. Because of its mediocre speed tier, Tapu Lele has options to invest some EV's on its bulk but it usually fails to change matchup against opponents it already loses to:
252 Atk Tough Claws Charizard-Mega-X Flare Blitz vs. 200 HP / 0 Def Tapu Lele: 322-381 (97.2 - 115.1%) -- 81.3% chance to OHKO
112 Atk Aggron-Mega Heavy Slam (120 BP) vs. +1 200 HP / 0 Def Tapu Lele through Reflect: 160-189 (48.3 - 57%) -- 91.4% chance to 2HKO
252+ SpA Magnezone Gigavolt Havoc (185 BP) vs. 200 HP / 0 SpD Tapu Lele: 295-348 (89.1 - 105.1%) -- 31.3% chance to OHKO
Tapu Lele has much better dual STAB than Tapu Koko and has good utility in Psychic Surge that blocks Fake Out (which would otherwise kinda screw defensive calcs over), Reflect, Taunt, etc. But its counters are widely known at this point and Tapu Lele can't do much back against them.
The most-reliable-checks:
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and others...
Why does this cancer exist? One thing makes Mimikyu great: Disguise. This lets Mimikyu efficiently run two of the most common sets: Swords Dance and Curse. Swords Dance set, while less common, is mostly guaranteed to have a turn to set up thanks to Disguise, and this lets Mimikyu boost its lackluster power and OHKO frail threats next turn. More viable set is the set involving Curse, which other Pokemon is not capable of running because they can't afford to survive a hit and have more than 50% of HP left to use such move. However, Mimikyu can safely use Curse thank to Disguise and spam Protect and Substitute to shut down anything that is slower and lacks recovery.
Now let's move on to this 'mon's drawbacks: Swords Dance is easy af to wall with zard x using Will-O-Wisp, or simply sending out fat 'mons like Ferrothorn and Celesteela. Even against less bulky opponents, 90 base attack leaves Mimikyu quite reliant on Z-move to do some actual damage, or it will have to be reliant on its Swords Dance boost while having fun surviving a hit with its 55 / 80 / 105 bulk without Disguise in this meta where Z-move spams are everywhere. So that leaves Cursekyu so much better option than setup variants of Mimikyu. Now what about Cursekyu? This is also predictable. Curse / Protect / Substitute is staple, and the other slot is Shadow Claw / Shadow Sneak or Play Rough with appropriate Z-crystal. I am not sure how other people checks it but what I do is simply sending out something with recovery or Rock Tomb. Probably this is a little complex option, but stuff like Donphan / Tapu Bulu / Electroweb Magnezone I use can check Mimikyu very easily. Just like Tapu Lele, Mimikyu has good STAB but it has 0 way to get past fat 'mons without a boost, and if it decides to run Swords Dance, it will mostly lose against most offensive 'mons. It has no way around its usual checks.
The most-reliable-checks:
Both forms of
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or any other fat mon that can slow Mimikyu
Gyarados is probably a 'mon that resembles the most of Kyurem-B by being fat, a powerful physical attacker, and having access to Mold Breaker. Unlike Kyurem-B, this thing has Dragon Dance which can be used to boost 155 base Attack even further while compensating its Speed stat.
But what really draws back this fish is its obvious movepool. What else it carries other than Waterfall, Crunch, Outrage, Earthquake, and maybe Ice Fang? This leaves Fairy and Grass-types entirely viable option to stop it. Though it involves usage of some underrated / underused / gimmicky 'mons, there are counters that are able to reliably check it. The most common ones I use are Tapu Fini, Ferrothorn, Tapu Bulu, and others. There are plenty of stuff out there like Tapu Koko / Lele, Mega Altaria, bulky zard x with wisp, Magearna, Venusaur, and others and
like all other 'mons I have mentioned, there is not a single way around these checks barring hax. (If you say Flyinium Z Gyarados I will actually murder you).
Also its bulk, even with Intimidate, requires some EV-shifting to be bulky enough to taken on the meta because of its common weakness to Electric and Fairy. This sometimes ends up compromising the power and changing outcome against the matchup against likes of zard x, Mega Metagross, Mega Mawile, and others.
The most-reliable-checks:
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Sorry about long lecture about other 'mons when you asked about Kyurem-B (I know that you know all these), I will finally proceed to explain Kyurem-B:
Ice and Dragon type. 170 Attack and 120 SpAtk or something similar is nowhere to be found in current 1v1 metagame outsides Kyurem-B. This already proves that Kyurem-B can run mixed sets unlike all 'mons I have mentioned except Tapu Koko. Literally no one in this meta likes taking BP 200 Subzero Slammer from this monster except the behemoths like Mega Aggron. On top of that, it has 125 / 100 / 90 bulk. 125 HP literally lets this Kyurem get 252 HP and live stuff like +1 Outrage from Gyarados, Moonblast from Tapu Lele by shifting the rest to appropriate EV's.
Outsides its bullsh*t base stats, what makes Kyurem-B a real pain in the ass to deal with is the access to Teravolt. If this guy didn't bypass ability, the following 'mons will be a check or at least have some chance against it at this point:
Teravolt says **** you to Sturdy users, obliterates Mimikyu, and tears Venusaur apart.
So countering this Kyurem-B means you have to find something that is bulky enough to take BP 200 move from 170+ Attack 'mon in the worst case scenario. Majority of Ice-resistant 'mons are unfortunately not able to stop this monster because Earth Power bops Steel-types...
0 SpA Teravolt Kyurem-Black Earth Power vs. 252 HP / 120 SpD Aggron-Mega: 158-188 (45.9 - 54.6%) -- 59% chance to 2HKO
(uninvested)
... while Fusion Bolt grills Water-types. Tapu Fini can't live Fusion Bolt at 252 HP / 252 Defense so I hope that clarifies Water-types cannot evade 2HKO from Fusion Bolt with the exception of Swampert-Mega.
So what can actually counter this kid? I would say the only 'mons that are able to beat it in most scenario are these:
: Stilll has to watch out for Weakness Policy
: If Kyurem-B uses sub on Yawn, life is over
: Don't let this guy get memed by Groundium Z.
Why not faster Dragons? They are going to be popped by Scarf sets.
Shortly, Kyurem-B has very limited counterplay which can be bypassed by investing on bulk, and base stats make this possible. Even when Marshadow was roaming around, Kyurem-B was one of the only stuff that could live SS7SS without dumping whole lot of EV's on bulk:
252 Atk Marshadow Close Combat vs. 252 HP / 0 Def Kyurem-Black: 384-452 (84.5 - 99.5%) -- guaranteed 2HKO
What differentiates Kyurem-B from Tapu Koko, Tapu Lele, Mimikyu, and Gyarados is the fact that Kyurem-B requires a counter to have
both the specific typing AND sufficient natural bulk, while Tapu Koko and Tapu Lele only requires an appropriate typing, Mimikyu requires a bulky mon with slowing move or an appropriate offensive 'mon, and Gyarados requires either acceptable bulk or dominant typing. This is very similar to Marshadow which required something really, really bulky to live SS7SS while having appropriate typing to take neutral damage from both of its STAB moves.
This is why I believe Kyurem-B is broken over Tapu Koko, Tapu Lele, Mimikyu, Gyarados, or anything else in the 1v1 metagame at this point. I was silently waiting for its decline as new megastones were being released, but with the exception of Aggronite release, nothing else seemed to make a change. I think the wait was long enough to see if Kyurem-B should really stay or should go. I hope this answered your question and clarified my overall thoughts about Kyurem-B and please feel free to point out anything that you do not agree with. Thanks.