The only counter is Toxapex and after chip damage, some sets can even take out Toxapex. Marowak and Tapu Fini are optional but have no reliable recovery and therefor can only switch in so much. It can have Poison Jab to deal with Tapu Fini and Clefable. Simply put, it's hard to switch into and limits teambuilding. After getting speed boosts, most scarf Pokemon can't outspeed to revenge kill it and people have to sometimes rely on priority. It can run Fightium Z Focus Blast which OHKOs Mega Metagross and can OHKO Mega Scizor. Almost everything is 2HKO'd after switching into non boosted Pheromosa, which is scary because almost nothing can come in AND outspeed it. Its movepool is unpredictable and you can easily lose a Pokemon you thought was a good switchin by trying to figure out its movepool.

You can easily bring it in to revenge kill things too and get a boost. That being said, a lot of things are scared of being ko'd and are forced to switch out, making it easy to get off Rapid Spins as well. It forces you to switch around a lot and everything starts to take damage. With hazards, its even harder as Zapdos can't come in after Rocks damage and live an Ice Beam and non-Scarf Latios is scared of Pheromosa in general. Tapu Fini, as mentioned above, has no reliable recovery and can only take so many hits and chip damage.

You can run it with Dugtrio to take out things like Toxapex and Tapu Lele to prevent priority. It can completely run through offense if it starts getting boosts and that has forced people to run Toxapex on offense just to have a safer switch in.

In conclusion, Pheromosa may be frail, but its offenses and speed make up for it by far. It's actually broken as you're limited to one or two Pokemon that are actually viable in the metagame as a probable switch in. For these reasons, I think Pheromosa should be banned from SM OU

 

Annoying Orange covered most of it. Pheromosa is very versatile, can outspeed pretty much everything and hits hard too. In my own experience, It's often taken 2-3 pokes just to play around Pheromosa and I often lose one that I need later in the match. If hazards are in play (Which is growing in usage because no one leads with Pheromosa), that damage adds up quickly and can put you well behind the 8-Ball if you make a bad guess at the set it's running. Toxapex is about the best bet against it, but Pheromosa users most times know this and have something ready for Toxapex, and then you're very well screwed because you then have to have the rest of your team ready to stop one pokemon, as well as the other 5. Granted Pheromosa is rather frail, and that's the primary selling point against banning it, many other pokes that are frail don't have the offensive power of Pheromosa, which balances it out rather well, really a little too well. I play SM Balanced Hackmons a little more than OU, and Pheromosa can be really tricky there too, just because of that amazing speed and power, the primary difference is that in the Uber/Hackmons tier, it can checked much easier, but is still a viable threat regardless. I say BAN Pheromosa from OU, I don't play Ubers that much, but Pheromosa gets a lot of usage in Balanced Hackmons and the high speed and power can be better checked while having even more versatility in the Balanced Hackmons tier. (First Impression w/ STAB + Close Combat w/ STAB can be very amazing in BH if done right). All in all, Pheromosa is a great pokemon, but a little too savage for the OU tier at the moment.

 

I'm mixed towards Pheromosa's ban.

She can't make the job alone, since Marowak-Alola and Toxapex can eats most attacks Mosa have to offer (Toxapex don't like eating a Z-Hyper Beam from a +1 QD Pheromosa though), but she's also a very threatening Ultra Beast (Pokémon ?) to face off. Here is some calcs to prove how powerfull this femininist UB is:

252 Atk Life Orb Pheromosa Poison Jab vs. 248 HP / 192 Def Tapu Fini: 177-211 (51.6 - 61.5%) -- 96.1% chance to 2HKO after Leftovers recovery (so Tapu Fini can't safely switch into Pheromosa in fear of Poison Jab)
252 Atk Life Orb Pheromosa High Jump Kick vs. 0 HP / 4 Def Metagross-Mega: 200-238 (66.4 - 79%) -- guaranteed 2HKO (and since Pheromosa outspeed him, she can U-Turn in his face in favor of Ferrothorn/Celesteela/Scarf Dorus)
4 SpA Life Orb Pheromosa Ice Beam vs. 0 HP / 0 SpD Landorus-Therian: 530-629 (166.1 - 197.1%) -- guaranteed OHKO
4 SpA Life Orb Pheromosa Ice Beam vs. 252 HP / 24 SpD Landorus-Therian: 520-614 (136.1 - 160.7%) -- guaranteed OHKO
4 SpA Life Orb Pheromosa Ice Beam vs. 0 HP / 0 SpD Landorus-Therian: 530-629 (166.1 - 197.1%) -- guaranteed OHKO
+1 224+ SpA Pheromosa Breakneck Blitz (200 BP) vs. 252 HP / 24 SpD Toxapex: 265-312 (87.1 - 102.6%) -- 18.8% chance to OHKO

But, Marowak is the biggest threat (and the safest one) to Pheromosa, being either immune or resistant to anything Mosa have to offer:

252 Atk Life Orb Pheromosa U-turn vs. 0 HP / 0 Def Marowak-Alola: 35-42 (13.4 - 16%) -- possible 7HKO
4 SpA Life Orb Pheromosa Ice Beam vs. 0 HP / 4 SpD Marowak-Alola: 66-78 (25.2 - 29.8%) -- guaranteed 4HKO
252 Atk Life Orb Pheromosa Poison Jab vs. 0 HP / 0 Def Marowak-Alola: 55-64 (21 - 24.5%) -- guaranteed 5HKO
2224+ SpA Pheromosa Ice Beam vs. 0 HP / 4 SpD Marowak-Alola: 66-78 (25.2 - 29.8%) -- guaranteed 4HKO
224+ SpA Pheromosa Bug Buzz vs. 0 HP / 4 SpD Marowak-Alola: 49-58 (18.7 - 22.2%) -- possible 5HKO

And Marowak OHKO her in return with Fire Punch:

252+ Atk Thick Club Marowak-Alola Fire Punch vs. 0 HP / 0 Def Pheromosa: 830-980 (293.2 - 346.2%) -- guaranteed OHKO

But I'm not a fan to throw Marowak-Alola in every teams just to counter Pheromosa (like I'm opposed to throw Knock Off just to counter Stall).

So I'm in favor to ban Pheromosa from the OU tier due to how little safe checks and counters she had but not because she's versatile and require predictions. That's a poor argument to ban anything.

PS: Calcs are done via the Damage Calcs from Showdown/Smogon.