Well, part of the reason the terrains went so underused in Gen 6 was that there was no auto-terrain ability, like there was for weather (eg. Sand Stream, Snow Warning, Drought, Drizzle). Remember when Ninetales was announced to have Drought as its hidden ability, and it shot right up to OU, along with Politoed, who got Drizzle? They were the first non Kyogre/Groudon users of Drought and Drizzle, and it made them downright indispensable to weather teams.
The Tapus are in a similar boat now. They're high end of OU, heavily used, because their abilities are unique and facilitate the other members of their team in many different ways, much like Sandstorms enhance Rock type pokemon's special defense by 50% for their duration, or how rain makes Swift Swimmers double in speed, similarly sun with Chlorophyll.
I think as Ultra Sun and Moon come out and Gen7's meta matures, we'll see the rise of hard hitting poison types, possibly excepting against Tapu Lele (the psychic/fairy one). Salazzle is a good counter in theory, but is too fragile, and Toxapex is too slow and passive. I've been playing around with a special based Nidoking; 295 spatk at level 100 isn't particularly great, but with Sheer Force and Life Orb you get a solid boost from both, and Sheer Force negates Life Orb recoil. It has Sludge Wave/Thunderbolt/Earth Power/Flamethrower, and it can deal with everything but Lele quite well.
Bulu is hit 4x effective by Sludge Wave, and its normal wallbreaking set is entirely neutral vs. Nidoking's Poison/Ground typing. Nido isn't frail, but it's not particularly bulky, either. A speed invested Bulu will outrun an uninvested Nidoking and likely KO with Wood Hammer.
Damage calcs:
+0 252ATK Tapu Bulu Wood Hammer (Grassy Terrain) vs 0HP/0DEF Nidoking: guaranteed OHKO (132-156%)
+0 252ATK Tapu Bulu Horn Leech (Grassy Terrain) vs 0HP/0DEF Nidoking: guaranteed 2HKO after Grassy Terrain (83-98%)
+0 252ATK Tapu Bulu Superpower vs 0HP/0DEF Nidoking: guaranteed 4HKO after Grassy Terrain (29.3-34.6%)
+0 252ATK Tapu Bulu Stone Edge vs 0HP/0DEF Nidoking: chance to 5HKO after Grassy Terrain (24.4-29.9%)
So yeah, not looking particularly good for Nidoking. If Bulu clicks Z-Horn Leech or Z-Wood Hammer (Bloom Doom) then it's a guaranteed KO no matter what, it just hits like a fuckin' truck. But Bulu only has 75 speed, and won't breach 249 speed at max investment, where the normal Nidoking Sheer Force Life Orb spatk set I use sits at 295. So, conversely:
+0 252SPATK Nidoking Sheer Force Life Orb Earth Power vs 0HP/0SPDEF Tapu Bulu: 55% chance to 3HKO after Grassy Terrain (34-40.5%)
+0 252SPATK Nidoking Sheer Force Life Orb Flamethrower vs 0HP/0SPDEF Tapu Bulu: 56% chance to OHKO (92-109%)
+0 252SPATK Nidoking Sheer Force Life Orb Ice Beam vs 0HP/0SPDEF Tapu Bulu: 56% chance to OHKO (92-109%)
+0 252SPATK Nidoking Sheer Force Life Orb Thunderbolt vs 0HP/0SPDEF Tapu Bulu: don't bother, it resists it, why did you click on this
+0 252SPATK Nidoking Sheer Force Life Orb Sludge Wave vs 0HP/0SPDEF Tapu Bulu: guaranteed 1HKO (294-345%)
I might write up how Nidoking deals with the other three Tapus in a bit; I'm testing that Shedinja idea using common Tapu movesets.
Shedinja can't do much more than 30ish% to any of the Tapus. Nature's Madness is Fairy type, so it deals no damage against Wonder Guard. Bulu, Fini and Koko's common sets can't touch Shedinja outside of taunting it, preventing it from Willowisping or taunting itself. Fini's Scald won't burn because it won't affect Shedinja. Tapu Lele is often run with Hidden Power Fire, which OHKOs Shedinja. Sadly, Shedinja doesn't get Endeavour, which might actually give it an offensive presence. So if it came down to a 1v1, everyone else is tired out, Shedinja could probably whittle down three of the four common Tapu sets, but it's beyond useless against anything besides that. And if the player's Tapus have deviated from regular sets, then it's a big gamble anyway.
Oh yeah, also; Tapu Koko's normal set as a pivot or with the Z-Wild Charge set, can't really do much against Nidoking. HP Ice 2HKOs Nidoking, but Nidoking OHKOs with Sludge Wave or Earth Power.
Tapu Fini frightens it out with Scald, Tapu Lele with Psychic.