I don't believe Garchomp fulls under any of these characteristics, and that Garchomp is OU, based on experience and statistics.
The first thing to clear up, is that when you decide to compare Garchomp to other sweepers to decide if he is uber or not, you have to remember how diverse Garchomp's sets are according to the usage statistics. Choice Scarf was the most used item, with slightly over 1/3 of the suspect community using it as their item of choice. Haban Berry was used 1/5 of the time, and Yache and Salac Berry were used 1/8th of the time.
But why is it, that when we compare Garchomp to other sweepers, we can assume Garchomp could carry all of these items? Isn't it true that any other sweeper could carry items or different sets to sweep? Infernape is a fine example, and although it will usually use Life Orb (almost 38%), 11 different attacks placed over 10% in usage.
Why did I even bother telling you this? It is because any Pokemon can change its moveset and attacks, just like Garchomp does on a regular basis, to better accomodate a team, while simutaneously being able to take on new threats, at the expense of losing to old ones. U-Turn, an attack used 16% of the time on Infernape, allows Infernape to scout Starmie and Latias switch-ins while hitting them for a significant amount. From Garchomp's point of view, resistant berries are a perfectly viable way to handle one of these two, (not both however).
Garchomp can run plenty of viable sets, and it could be the best at all of them in his respective areas, but running new sets to combat Pokemon can leave him vulnerable to old threats, just like every other Pokemon. Let's also not forget how often Garchomp was used. 79% of the time, and this has plenty of reasons and effects on the metagame. Garchomp is being used so much because it is an excellent Pokemon and it is a Pokemon that is highly debated right now. Not to mention that using it will grant a user higher SEXP. This increased usage results in us remembering Garchomp more than other Pokemon who will run different sets to combat new threats, but this is not an entirely new strategy.
Here is an example of what I am talking about. Sub+Salac Berry Garchomp is a good Garchomp set, that plays a new role, on of luring in Latias or Starmie, and hoping to become faster and set up a sweep. But just like any other sweeper, Garchomp gains new weaknesses when attempting this. Skarmory and Bronzong will be able to handle anything Garchomp throws at them. Pokemon such as Magnet Rise Magnezone can now avoid Earthquake with some correct prediction or in a decent situation and won't have to worry about Fire-type attacks. Yes, Sand Veil is an excellent ability and can help make things go much easier for Garchomp, but by that same token, relying on it to bail you out as you substitute (with no healing) is playing russian roulette with one of your "prized sweepers".
Haban Berry Garchomp, usually decides to forgo Substitute and put Fire Fang or Fire Blast on instead. Yes, this makes it much easier to handle Bronzong and Skarmory (despite the fact that they can still Explode or Whirlwind Garchomp, as not only is +2 not a given, but also because Skarmory or Bronzong can ev themselves to survive these situations.). And without using Yache Berry or Substitute, Pokemon such as Mamoswine, Starmie, Weavile, or whatever other Ice-type attack you throw at Garchomp, will be able to handle, and potentially kill Chomp that much easier.
So yes, Garchomp can run different sets to combat different Pokemon. And factoring in all different sets and assuming you can get a Sand Veil miss, Garchomp may be able to handle every single Pokemon in OU. But so can every other sweeper, assuming they get their set-up, or their support, and whatever else Garchomp may have.
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Now the real issue at hand; What can actually stop Garchomp? Well as I just pointed out, Garchomp has issues with different Pokemon depending on his set, not to mention some universal counters as well. What makes me somewhat angry is that more Pokemon aren't being tested or experimented to handle Garchomp. An example would be Suicune and Manaphy. Both of them have Reflect at their disposal, and it is never used on Suicune (and perhaps never used on Manaphy either). Suicune has Roar, a move only used 20% of the time. Both Pokemon can heal fairly effectively, and hit Garchomp with super-effective Ice Beams.
Onto more "unused checks and counters", Cresselia's Reflect is only used less than 1/3 of the time. Cresselia herself is only used 2.4% of the time. I know people will come in and say "But Cresselia can be easily set up on by Tyranitar or Scizor!". Well sure, but this is where playing smart and using a team correctly plays to your advantage. No Pokemon can handle threat, and while Cresselia does handle a large amount of them, Tyranitar and Scizor are not some of them. But add a Machamp, or a somewhat fast Tyranitar with Low Kick and Fire Blast. You could even turn those two Pokemon into somewhat "Chomp checks", by abusing Machamp's No Guard and investing into Defense, or by even using a Choice Scarf to outspeed Garchomp and use Dynamicpunch or Ice Punch.
This is why experimentation is so important in suspect, and why it should be documented. Have you played with something useful or something you thought could have potential but failed? Just post about it so we have a reference sake.
Onto more traditional methods of dealing with Garchomp, Life Orb and Choice Specs Latias can usually OHKO through Haban Berry with Draco Meteor. Latias can switch into Garchomp somewhat easily, although it must watch out for Dragon Claw or Outrage at any time. Is this unfair to you? My argument here is that Latias has those same problems when switching into Infernape, what if it U-Turns to a Scizor or a Tyranitar? What about Skarmory who decides to Brave Bird or Whirlwind, instead of using Spikes as you predicted? These are risks of Pokemon, otherwise the game wouldn't be the same. Scizor's Bullet Punch can 2HKO Garchomp, and puts Garchomp almost useless. Sure Scizor could miss thanks to Sand Veil. But what if, Scizor got a bad damage roll on Salamence that allowed Salamence to come out on top? I have seen this happen before. Do we just give up? The answer is no, we build to play around these situations, which happen all the time, but are so much more prolific with Garchomp.
Celebi can Perish Song and throw up Reflect on Garchomp, and invested in Defense could force Garchomp out, if it can't ko before you Perish Song. Mamoswine, Starmie, and Weavile can all kill you with Ice Shard or Ice Beam if you aren't using Yache Berry. If you are, this leaves you open to a Scarf Salamence, an opposing Garchomp, or even other Latias.
Garchomp is an excellent Pokemon. Will never deny that. But to suggest that Garchomp can not be handled is ridiculous. Garchomp may be an extraordinary sweeper, but other Pokemon can still handle Garchomp, just as they can handle other sweepers such as Lucario and Infernape, and most of the time, it all depends on the moveset.
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I hope people feel that what I had to say was valid and hopefully not a waste of time. These are just my feelings on the matter though.