You may be wondering why some of the changes were made from the
ORAS viability rankings, so here is a summary of why notable changes down to B- happened or didn't happen.
What moved:
Vullaby: A+ -> S
Vullaby has always been effective at what it does, but now it can effectively do so many more things. It'll be tough to figure out which Vullaby set you're up against because it's so splashable. Pawniard Knocking Off Vullaby may be just what the Z-Mirror Move set needs to sweep. Try revenge-killing Vullaby with Cutiefly and you may be met with a Scarf Brave Bird instead.
Cutiefly: A+
Despite all attempts to check it, Cutiefly has still managed to cement itself as a top thanks to Quiver Dance, Baton Pass, and all the right coverage/supporting moves. Whether you're using it to sweep directly, support a Porygon waiting in the wings, or even check things with its great typing, Cutiefly can be hard to pass up.
Mudbray: A+
I'm sure you already know how good Mudbray is, so I'll actually explain why it's not S. Mudbray doesn't have much of a defensive niche, meaning it's mostly included in free slots. In addition, unlike any other S rank, it is actually possible to hard check in a single teamslot, so while a very strong Pokemon, it doesn't feel as omnipresent as the current S-ranks do.
Abra: A -> A+
Now that Shellder has competition for top sweeper, Sashbra is back in business! With the presence of Cutiefly, Psych Up + Psyshock is now so much more than a gimmick, in addition to preventing Abra from becoming CM Gothita setup bait. Abra also checks Scraggy and enjoys the rise in Poison-types.
Gothita: A -> A+
Gothita synergizes fantastically with many of the best sweepers in the metagame, especially Cutiefly and Scraggy. It does a great job at eliminating troublesome Poison- and Fighting-types, and the CM variant all but guarantees the removal of Spritzee. A lot of people consider GothFly and ScragGoth even stronger than ShellDig!
Scraggy: B+ -> A+
The two things that troubled Scraggy the most last gen were Fletchling and Fairy-types. Well, Fletchling has been heavily nerfed, and the most popular offensive Fairy-type, Cottonee, can no longer affect Dark-types with Prankster moves, leaving it susceptible to Poison Jab. Meanwhile, the ever popular Gothita traps Fighting- and Poison-types. As long as Scraggy avoids revenge-killers, it pretty much has free reign with its fantastic STABs and coverage, which is not a pretty sight.
Staryu: A- -> A
Staryu hasn't actually gotten better - it's still as passive and Gothita-weak as ever. But the other hazard removers are in an even worse state, and in Vullaby's case, almost feel like a waste of a great offensive Pokemon. Staryu is often stuck onto a team purely for its utility, and as a result, ends up as one of the most splashable Pokemon around.
Doduo: B -> A-
Doduo already had a strong Knock Off and an absurdly powerful Brave Bird; now it also has great physical coverage in Jump Kick and less competition from Fletchling. It may be the frailest of the high tier birds, but Doduo is also by far the most immediately threatening.
Dwebble: B+ -> A-
Hazard removal is tough to come by, which means hazard stacking is great! Dwebble has always been the poster child of hazard stack, but now it has a more reliable bulky Weak Armor + Eviolite set to work with. Metagame shifts have rendered Dwebble arguably more effective than its main competition in Omanyte, so as the premier offensive hazard stacker in LC, Dwebble's position in the A ranks is secured.
Munchlax: B+ -> A-
Despite barely clinging to B+ last gen, the increased presence of Cutiefly and weather has made Munchlax's previously rather small niche as a Special wall much more important. Phazing is very useful with the increase in hazard stack and Baton Pass; thanks to its great Special bulk, Whirlwind Munchlax can cause quite the headache for quickpass and full Baton Pass chains alike.
Onix: B+ -> A-
Onix was already on the verge of rising before; soft checking Cutiefly and having Gothita help to shut down Fighting-types only serves to amplify its niche.
Rufflet: C+ -> A-
Thanks to the addition of Brave Bird to its movepool, Rufflet is no longer held back by a lack of strong Flying-type STAB compared to other birds. If you can't decide Vullaby's bulk, sweeping potential, and access to U-turn or Doduo's raw power, Rufflet gets some degree of both. However, this comes at the significant cost of lacking Knock Off.
Mareanie: B+
Though not as influential as Cutiefly or Mudbray, Mareanie has still made an impact on the metagame thanks to Regenerator, fantastic set of resistances, and access to key support moves in Haze and Toxic Spikes. Being weak to trappers has definitely hindered it, but at least it's not setup bait for CM Gothita.
Archen: A- -> B+
One of Archen's main uses was checking Fletchling, which is much less useful now, and Rufflet, Doduo, and setup variants of Vullaby are able to break through Archen much more easily. Archen also dislikes the increase in Mudbray, Staryu, and Scarf Gothita.
Drilbur: A- -> B+
In addition to Diglett, Drilbur now has Mudbray to compete with for a slot as the Ground-type on a team. It also matches poorly against Scraggy, Staryu, Pumpkaboo, and stronger birds.
Omanyte: A- -> B+
Although it was approaching mid A in gen 6, Shell Smash Omanyte now has serious issues setting up without Fletchling in the tier; Vullaby frequently has its offensive stats boosted while Doduo and Rufflet have strong Fighting coverage. It also really needs Modest to get the KOs it wants, meaning it only hits 26 Speed after a Shell Smash, but the 27 and 28 Speed tiers have been inflated, leaving Omanyte more easily revenge-killed. Whirlwind Munchlax makes matters worse. The hazard stacking set now has serious competition from Weak Armor Dwebble, whose set of weaknesses often hinder it less.
Timburr: A+ -> B+
Talk about a fall from grace - despite a great new partner in Grimer-Alola, the metagame is extremely unfriendly to Timburr right now. Bulk Up is by far its best set, but being walled by Fairy-types is a much bigger deal with Cutiefly around. Mareanie completely shuts down Timburr with Haze, and Foongus is running Clear Smog more often than before to deal with Cutiefly and Baton Pass. Although Fletchling is gone, Doduo and Rufflet are even more threatening to Timburr. Vullaby and Gothita being even more prominent than before only makes matters worse.
Pumpkaboo-Super: B -> B+
Not everything new to B+ was dropped down to it! Though Pumpkaboo is still quite passive, its defensive niche is more important than before. Giga Drain allows it to hard check Mudbray, and spinblocking Staryu makes Pumpkaboo a good teammate for hazard stacking.
Torchic: B -> B+
Torchic was already one of the better mid B Pokemon, but cool new quickpass recipients and Cutiefly strengthening Baton Pass chains makes bulky Torchic even better.
Grimer-Alola: B
With strong STAB attacks and Poison Touch, Grimer-A has solid offensive presence alongside its utility in trapping the hugely important Gothita, Abra, and Gastly. However, it is almost completely shut down by Mudbray and is trapped by Diglett; in addition, its lack of recovery and few resistances mean it gets worn down quite quickly despite its decent bulk.
Sandshrew-Alola: B
Hail has turned out to be surprisingly effective, and Sandshrew-A is its main abuser. Despite its gaping weaknesses, Sandshrew-A remains quite potent under Hail thanks to its excellent coverage, good Attack, and access to Swords Dance.
Skrelp: B+ -> B
Skrelp still has its typing and power, but Gothita, is more widespread than ever, including its troublesome Calm Mind set, and RestTalk Skrelp now directly competes with Mareanie.
Amaura: B- -> B, Snover: C+ -> B-
With the newfound viability of Hail, Hail setters are of course better than before! Amaura is generally the first choice for this role thanks to its coverage and access to Stealth Rock to pressure switch-ins, but Snover's priority useful typing offers its own advantages.
Aipom: B+ -> B-
Once one of the scariest wallbreakers in the metagame, Aipom has met its match in Stamina Mudbray; no longer is Aipom almost impossible to safely switch into. Weak Armor Dwebble also sets up hazards in Aipom's face. Mienfoo, Gothita, and Vullaby are now frequently donning a Choice Scarf, meaning Aipom is forced out by revenge-killers more often.
Buneary: C+ -> B-
With all the hype surrounding Conversion Porygon and Mirror Move Vullaby, one big z-move related buff is often forgotten - Buneary's Splash! Outright sweeping sets may have trouble differentiating from Zigzagoon, though +3 Return and Quick Attack is no laughing matter. Splash Buneary's main draw is its access to Baton Pass.
What didn't move:
The ORAS S-ranks
Pawniard is the closest to moving down due to the Sucker Punch nerf and Vullaby more frequently running anti-Pawniard coverage, but its ranking is still primarily because of its numerous resistances and incredibly threatening Knock Off, both of which it has kept. Mienfoo hasn't been as badly hindered by Cutiefly and Gothita as Timburr has; it's always had a free coverage moveslot which is now more often than not Poison Jab or Stone Edge, and U-turn, Knock Off, and decent Speed give Mienfoo plenty of counterplay against Gothita. Its Scarf set has actually gotten better due to being able to revenge-kill Scraggy and not having to deal with Fletchling as often. Diglett remains relatively unchanged, revenge-killing Eviolite Pawniard more easily but having its own priority move slightly weakened. Porygon has been outright buffed, as it now has Z-Conversion to add to its already immense arsenal.
Foongus
The metagame shifts have affected Foongus a lot, but the net effect is more or less neutral. Gothita becoming more popular is a big blow to Foongus's viability; on the other hand, Foongus gains a niche as a repeated check to Cutiefly, Fletchling has been nerfed, Mienfoo rarely runs anti-Foongus coverage anymore, and being able to absorb Toxic Spikes is slightly more important than before.
Magnemite
Mudbray may ruin Magnemite's day, but at least it's not as bad for Scarf Magnemite as the now uncommon Berry Juice Drilbur. Meanwhile, Cutiefly, Mareanie, sweeping variants of Vullaby, and to a lesser extent Sandshrew-Alola all join the list of Pokemon that SturdyJuice Magnemite soft checks.
Cottonee
Despite not annoying Scraggy and Nasty Plot Vullaby as much anymore, as well as being hampered by an increase in Poison-types, Cottonee still supports Shellder as well as ever, and appreciates enemy Shellder being less likely to carry Ice Shard. It also enjoys not having to deal with Fletchling.
Ponyta
Not lowering Ponyta is very debatable; being walled by RestTalk Mudbray, Staryu, Munchlax, and Mareanie really sucks, and Vullaby often being Nasty Plot doesn't help, either. On the other hand, checking Cutiefly is very useful, and it is fairly irritating for Sun and Hail as well.
Croagunk
Croagunk doesn't mind Cutiefly as much as other Fighting-types due to its Poison STAB, and unlike Timburr, it has some counterplay around Gothita with Bulk Up Sucker Punch. Sucker Punch is also much more useful against Doduo and Scarf Rufflet than it was against Fletchling. Struggling so much against Mudbray definitely hurts, but it is mostly offset by being able to check the more frequently Zen Headbutt-less Scraggy quite well.
Hippopotas
Hippopotas may now compete with Mudbray as a bulky Ground-type, but the increased importance of Whirlwind, being able to hard check Sun, and cutting Sandshrew-Alola's sweep short all help to ensure that Hippotas's niche isn't any smaller than it was before.