PowerWhip-less versions are smothered by SubCM Keldeo, Genesect fucks all Ferrothorn, Tornadus-T 2HKO's, and Thundurus-T can set up and then KO with +4 HP Ice, while being resistant to Gyro Ball and immune to Thunder wave. Landorus-T can't do as much, but it can still hit hard with EQ, and U-Turn to avoid Leech Seed. Because nearly every single team can't function without one of the above pokemon, there is always a counter to Ferrothorn. This strays from the idea that A tier 'Pokemon who can sweep or wall significant portions of the metagame' a bit.
Going to go through your list here. Firstly, most Ferrothorn run Power Whip, so you check most Keldeo pretty hard, Genesect doesn't really "fuck you", it has to flamethrower if it wants the kill, otherwise it U-Turns out, takes SR switching in, 12.5% from IB, and you can Leech Seed (since your slower) whatever he U-Turns into. If they have a grass type like Breloom, then fine, you survive the U-Turn, and set up a layer of spikes, and now Gensect is taking SR + 1 layer of spikes next time it switchs in. U-Turn is going around 40% ish from experience so you are still high enough to set up a second layer if you wish (with leech Seed extending this) so yea, you are kinda wrong here. Tornadus-T can 2KO sure, but it fears Thunder Wave (which will cripple it) or Gyro Ball (which also cripples it). I don't know how Thundurus-T is getting to +4 SpA, and not choosing to simply KO with Focus Blast. I assume you are thinking of Double Dance Thundurus-T, which is poor vs Ferrothorn. Ferrothorn can do whatever it likes in those 2 free turns. It can Leech Seed and stall you via protect, it can set up 2 layers of Spikes (its now done its job as I can bring in my revenge killer resulting in a net gain), or it can attack, taking advantage of the fact you prolly took 25% switching in, are running LO, and I can spam Gyro Ball / Power Whip for the 3KO or whatever it is.
Landorus-T gets spiked on, or stalled with leech Seed / Protect, I don't really consider it "trampling Ferrothorn" but idk, maybe you play shit Ferrothorn users. You don;'t even avoid leech Seed, since you are faster, so if you U-Turn, you took 12.5% from Iron Barbs, AND your switch in can get Leech Seeded sooooo. Your final point is that "every team has a counter for it" which is true, but that doesn't make it shit. Hippowdon, has to face the possibility that pretty much every member of a rain team can deal with it, but this doesn't make it any less useful, because (if your not an utter retard) your not bringing it in on something that 100% beats it (like im generally not bringing it in on Heatran), you are bringing it in on say, Starmie, or Politoed, heck, I bring Ferrothorn into Tyranitar (if I can predict the Tar switch) because it taking over 50% is big in winning the weather war, and its forced out after getting up SR, giving me a free layer.
I'm an avid user of SubCM Jirachi, and I've seen multiple times how many teams get raped because their players think Ferrothorn can beat set up sweepers with Gyro Ball
Ur facing dumb users of Ferrothorn then, also, why are you so keen to assume Ferrothorn only has Gyro Ball for its only attack? (Not that you beat Jirachi if you run Power Whip)
As much as I hate to say it, defensive teams are getting harder to play with, especially since BW2 didn't really help stall that much.
And yet, ironically, Hippowdon, an exceptionally common "stall mon" or "defensive team mon" is A tier, prolly because, you don't have to run stall to use it. Hippowdon is good on balanced teams (more hyper offensive teams
can (but not always) suffer from a loss of momentum with it) and yes, Ferrothorn does not need to be used solely on a stall team for it to be good. It works fine on a balanced team, its does its job reliably, and unless someone can make a halfway decent post that convinces me to keep it in B tier, ill keep pushing it towarsd A tier.
Keep Volcarona in B Rank. Is it a deadly sweeper? Hell yeah! Is it easy to set-up? Hell no! Between SR that makes Volcarona unable to set-up against almost anything offensive, the fact that offensive teams are everywhere and Volcarona needs to be bulky in order to set-up, which means using a sun team, and the fact that Terrakion and Keldeo are everywhere, Volcarona just can't set up and sweep as easy as one would like to think. It requires too much support to be in A Rank.
The support you mentioned, is pretty much limited to "run a spinner" although its true that sun support really brings it out on Volcarona. If SR is not up, Volcarona can set up very easily, forcing out many pokemon weak to fire, or bug, heck, its a good switch in to Genesect if Rocks are not up, so thats nice. I like how you brought up Keldeo, which actually struggles vs Volcarona if sunlight is up, and yea, both Terrakion and Keldeo hate Giga Drain so your point is moot (also Sun teams run Dugtrio for Terrakion / Heatran anyway). iv actually used Volcarona a fair bit, and it does sweep a lot, it really should be A tier. (Heck, it prolly sweeps better than Latias anyway).