um taunt beats chansey, but you don't need roost to beat amoong. Natures madness on the switch in and u-turn out, terrain will prevent spore on the switch in and gain momentum for the team. And what, since it doesn't beat every pokemon its bad? Every pokemon in OU will have counters depending on their sets, if they didn't, they would be ubers.
Gliscor can be u-turned on.
And did your really say m-sab is a counter WTF LMAO:
252 SpA Zap Plate Dedenne Thunderbolt vs. 252 HP / 144+ SpD Mega Sableye in Electric Terrain: 153-180 (50.3 - 59.2%) -- guaranteed 2HKO
nice counter bro.
oh, Sableye is more frail than I thought. Anyways, you misunderstood me. It's a stallbreaker, it's supposed to be uncounterable to stall teams, which is why I never said that, for example, any ground type potentially counters it. Of course they do, but that's not the point. The point is that against more offensive teams, u-turning against counters is valuable, but against stall it's not. Finding free turns against stall is about as easy as finding a mcdonalds in an american town, the entire playstyle is intent on giving the opponent as many free turns as they want, with nothing to do with them. Firstly, let's frame this argument for the best. Roost is worthless, if you lack nature's madness you can do nothing but u-turn on ground types and Venusaur, which is worthless against full stall, and if you lack Taunt, Chansey beats you, again you're worthless vs. Stall. So, the only real set here is Thunderbolt / U-turn / Nature's Madness / Taunt. With this you can think of damaging or outstalling these threats. Now lets talk about the limitations of the set.
So, I've gone over the collective threat list, and I think you are right. The only proper counter to the set that is viable on full stall teams, is Gliscor. I would say that Mega Venusaur, Mega Charizard X, Quagsire, Gastrodon, and Hippowdon are also a serious threat, as they can simply keep coming in and using their recovery move as you switch out, accomplishing nothing on either sides, which is usually favourable to stall. Of course, you could Taunt, but then you risk being OHKO'd by Sludge Bomb/Earthquake/Flare Blitz. This is a two way street, as if you predict correctly, your opponent is in serious trouble. Also, Synthesis has a mere 8 PP, so simply continually forcing Venusaur in, and forcing it to Synthesis without gaining anything could work, assuming you have perfect hazard control and you can easily get Koko in for free.
So, despite these hiccups, I'll definitely agree that the stallbreaker set is legitimately threatening to stall. Is losing to Gliscor and labouring to get past Venusaur, Quagsire, etc. too much to make it unviable? Why use a stallbreaker set instead of coverage moves, if you can't break stall? Well the answer really only lies in usage and how well you can read the opponent.
Now that we've established that Koko can potentially break apart stall, lets ask ourselves, is it worth the teamslot? Because lots of stuff beats stall, like SD Life Orb Haxorus (yes it can get past Skarmory), but is kinda deadweight vs. more offensive/mixed teams. So how does this Tapu Koko set fair vs offense? I think the best place to start is the compare it to how offense performs, and then, considering how much better the stallbreaker is vs. full stall, do a cumulative usefulness analysis.
Here are the three sets I will directly compare
Tapu Koko @ Life Orb - 252 speed / 252 SpA / 4 Attack / Naive - U-turn / Thunderbolt / Brave Bird / Hidden Power Ice
Tapu Koko @ Life Orb - 252 speed / 252 SpA / 4 Attack / Naive - U-turn / Thunderbolt / Dazzling Gleam / Hidden Power Fire
Tapu Koko @ Magnet - 252 Speed / 252 SpA / 4 Attack / Naive - U-turn / Thunderbolt / Nature's Madness / Taunt
Both have the same playstyle. Both seek to attack with strong Thunderbolts, pivot out of things it can not beat, and beat the counters they potentially can with their last two moves. Both sets will beat whatever each other beats with Thunderbolt, and u-turn is the same. So the question is, who fairs better against problematic Pokemon with their last few attacks. Here's a threat list, taken from the ORAS viability rankings to show what each set can and can not beat.
Landorus-T, and Landorus - Offense performs better, because they can OHKO or 2HKO. Though stallbreaker is not too bad as its able to turn its momentum into a 50% chunk taken out of Lando this however only applies to the early game. Should it be later, Scarf Lando can come in against stallbreaker Koko, with as little 20% hp, and be get a turn to attack, unless you can u-turn as it comes in and bring in something faster.
Garchomp - Offense performs better being able to KO. Stallbreaker is turning its momentum into a chunk of damage vs. chomper, but does it either give it a turn to set up rocks(pending a risky taunt!)/attack, or take nearly no damage and deal a ton of passive with skin+rocks. If Garchomp is low on HP you pretty much can't hurt it, and if you do you're going to do more to yourself.
Excadrill - Again, another offensive ground type that the stallbreaker can do little against. Yes, hitting excadrill with Nature's madness early in the game does a nice chunk, you don't have a way to really hurt it if its low on HP, meaning excadrill can quite readily take advantage of you. Unless you have a good check to Scarf Excadrill that's up, u-turning as Excadrill comes in may not be productive. This being said, HP Ice Koko is pretty weak against Excadrill too. HP Fire variant wins here.
Ferrothorn - HP Fire wins. Stallbreaker actually loses 1v1 (0 Atk Ferrothorn Gyro Ball (150 BP) vs. 0 HP / 0 Def Tapu Koko: 205-243 (72.9 - 86.4%) -- guaranteed 2HKO). Though Nature's Madness does a good chunk, Leech Seed and Protect spam will be rather good at sustaining Ferrothorn. Simply hitting it with Nature's Madness everytime it comes in doesn't wear it down that well.
Gliscor - HP Ice wins. Gliscor beats stallbreaker and hp fire.
Chansey - Only stallbreaker can touch Chansey
Kyurem B - Stallbreaker's ability to Madness + U-turn, rather than just U-turn makes it a better here actually.
Venusaur - Brave Bird can beat Venusaur, but, PP stalling with the stallbreaker, or going for a crippling gambit, is possible.
Hippowdon - Stallbreaker can make a gambit vs. Hippo to cripple it, though it is risky. Nature's Madness on the switch, then either U-turn or Taunt. Predict correctly and win, wrong and die.
Gastrodon - Pretty much the exact same thing as Hippowdon.
Quagsire - Another Hippowdon. Though Dazzling Gleam can 2HKO, no one would ever use Quagsire outside of full stall, and in that case Chansey would beat you.
Charizard X - The final Hippowdon-like.
Of these threats that require specific moves to beat:
Brave Bird / HP Ice beats: Landorus, Garchomp, Venusaur, Gliscor
Dazzling Gleam / HP Fire beats: Landorus, Garchomp, Excadrill, Ferrothorn
Stallbreaker beats: Chansey, and has a chance of beating Hippowdon, Quagsire, Gastrodon, Charizard-X.
I think I can distill it like this. The offensive sets are obviously better at dealing with offensive threats, and defensive Pokemon you're more likely to see on "balance" rather than full stall such as Mega Venu, Lando-T, Gliscor and Ferro. After careful analysis it's fairly clear that the stallbreaker does have some merit. Its only true counter is Gliscor, which certainly wont be on every stall team. However it is quite clear there are a lot of stall Pokemon that Tapu Koko must rely on a fairly risky Taunt or U-turn choice, so its ability as a stallbreaker is imperfect.
I would say all three sets have their merit, and its really up to the team built around Koko to dictate its moveset. What threatens you more? Gliscor and Venusaur? Excadrill and Ferrothorn? Or is it full stall?