Specs Tangrowth gets 2HKOed by CB Outrage and takes a hefty load from SD Outrage, another 2HKO too, and requires Specs to even OHKO Garchomp, which comes with a lot of limitations.
Revenge-killing is not a viable strategy since Garchomp has already done its damage by then, and as pointed out is blocked by Yache Berry. If Infernape was basically used as much as Garchomp, its counters would rise but it would be hindered by its limitations - poor defenses and not so great typing, saving it from being banned. It was the same issue with Blissey / Tyranitar in ADV, they were certainly used around as much as Garchomp but they did not have as much power or the supposed uncounterability that Garchomp is supposed to have.
In every situation we are assuming optimal conditions for the player who uses Garchomp, to attempt to prove that Garchomp can be stopped by other pokemon barring extreme hax. If pokemon A needs Garchomp to run 0 HP/Def IVs and a -Def nature to counter it is not a viable counter.
I'm not talking about the choice bander. I don't really get why people say "Oh Cresselia can counter most SDchomp but CBchomp owns it with Crunch or Outrage!" Yeah, Suicune can counter physical Salamence, but oh wait it could have Choice Specs Draco Meteor! Also, regardless of how viable you think choice specs Tangrowth is, it is a counter despite taking a lot from SD Outrage. It actually is a good set, might I add.
And wait, revenge killing is not a viable strategy? Tell that to usk and ipl and the several other battlers that have used such a strategy to great success. I myself have. Garchomp isn't the only one that can do damage to those teams either - CB Hera can show up and just demolish anything, and then Deoxys-S comes in and Heracross switches out. Yeah Heracross has already done its damage, but it's not like taking out a slow wall where it can open up tons of sweeps for other sweepers on your team; Sweepers on these teams are not relied upon for much of a defensive workload, and switching anything in on a CBcross that did did rely on would just be playing badly.
I don't really know where you got the 0 IVs thing from. We're being perfectly fair with what does and doesn't counter Garchomp. Also, there are revenge killers that can bypass Yache Berry. If you've ever used scarf dragons in Salamence, Dragonite, and Garchomp, you know what I mean. They're very good too. There's also ScarfTran or a CB Ice Punch from Weavile. Garchomp rarely gets in with all of its health.
As for your more recent post...
Yes usage is related to power, of course it is. However, usage does not define power. Power doesn't even define usage in all cases.
Also Garchomp can switch into Tyranitar and Lucario and Electivire without a speed boost and win every time, but it's not like Blissey countering Starmie where Blissey is always going to come out on top with almost all of its health. Tyranitar's CB Crunch effectively shuts down SDchomp's ability to sweep. LO Close Combat, if Lucario predicts the switch-in, will also take off a good portion of Garchomps health (more than half). If you want Evire to shave off 50% of your health and take away your Yache Berry too, that's fine by me. Heatran? Cool, let's eat a Dragon Pulse or HP Ice. Even if Garchomp predicts a Fire Blast or earth Power, he won't exactly take the hit like nothing. Blissey? Toxic and Ice Beam are both fairly common, though I actually agree that Thunder Wave is the better choice for Blissey. Infernape? It almost always carries Hidden Power Ice, making Garchomp a really terrible switch-in.
You also said this which caught my attention:
"I think the +Ubers side is arguing that Garchomp can sweep OU with its most common set, which is obviously a quality that needs to be noted."
Sweep OUs? Garchomp might be fast, but it isn't fast enough to sweep OUs. What revenge kills it:
Mamoswine
Weavile
Scarf Salamence
Scarf Garchomp
Scarf Dragonite
Deoxys-S
Scarf Gengar
Scarf Infernape
Scarf Alakazam
CB Donphan (survives Outrage and takes 90% from Garchomp with CB Ice shard, 2HKOing with or without Yache Berry).
Scarf Heatran
Scarf Lucario
Scarf Gyarados
Metagross (assuming it's Outraging)
A lot of those threats are ridiculously common. Scarf Gar and Deoxys-S are seen every day on Shoddy. The others aren't as common but it's not like they aren't used (outside of CB Donphan). You have a lot of options to revenge kill Garchomp and end its sweep. A lot of those pokemon (Gengar, for example) can revenge kill it without a Choice Scarf and opt for a Choice specs or Life Orb set. Alakazam does similar (and Specs Psychic does ~70% to Garchomp while HP Ice, specs or not, OHKOs). Its speed just isn't enough to carry out a sweep in today's fast and powerful metagame.
Garchomp also has to switch in on the threats of today. It's bulky one cannot deny, but switching in on things it supposedly counters can damage it a lot as I mentioned earlier.
"If usage does not determine Uber status, there isn't much reliable evidence to prove that a pokemon should be Uber in the first place."
Usage =/= to viability/power. OU/Ubers is decided by power, not usage. So then yeah, we don't have any stats proving either side is "right." There were no statistics that
PROVED Wobbuffet was uber, but it was banned because it was a detriment to just about every in the metagame. I haven't seen Garchomp "destroy the metagame" like I've heard people claim. We have a whole selection of revenge killers at our disposal to beat it and a select few pokemon who counter it.
EDIT: Yeah AC, it actually would lock you in. Just so ya know ;)