I think the biggest issue here is "defining what you think is Uber". Allow me to dig into Smogon's Philosophy.
Here is the reference.
Let's read what this says:
"The "OU metagame" is the result of a search for a balanced game, where player skill, teambuilding skill, and a certain amount of luck combine to execute victory. The "OU metagame" is in no ways perfect, but it should be pointed out that 99% of multiplayer games are often plagued by imbalance and the resulting "tiers", and it is fortunate that Pokémon's detailed depth, combined with the intelligent minds of its players, working to prevent various abuses, is capable of producing a diverse and enjoyable arena."
This phrase is actually important to the discussion. It states many things that we look within the OU tier. This being:
- A Competitive Metagame
- Balance, with some limitations (hax always exists kids, sorry)
- An "enjoyable" arena
These are pretty basic to what we seem to look for. If we look at these three categories, you can actually conclude a lot of things about Garchomp being Uber. Though, now you hit fine lines like Deoxys-S. How does it contribute negatively to a competitive metagame? Is the "arena" still enjoyable despite its presence? Does it require luck to kill? We can obviously answer to number 3 question as a "no". As for the other two, it is tricky.
Let's try to answer question #1. How it contributes negatively to a competitive metagame? Let's dredge into answering the question a little differently; i.e. how it contributes
positively. For starters, its 180 base Speed allowed it to not require a Choice Scarf to be used as a "Revenge Killer". In some people's eyes, this actually seems like a positive aspect to the competitive metagame. There is nothing more frustrating to others having to gimp their Pokemon's potential only to stop others such as Lucario or Salamence with a Choice Scarf slapped onto their Pokemon. When Garchomp was dominant and Shaymin-S entered the scene, Deoxys-S contributed positively to the metagame as a revenge killer because it provided the player with "checks" to these threats. You can also include others, such as Salamence, into the picture.
Now let's look at the negatives. So first we hit Leads. Now, while many people believe Deoxys-S as the "prime lead", one of its major reasons for being "Uber" was due to its insane Speed allowing it to lay Stealth Rock with little penalty. You had a 50% chance of stopping Stealth Rock without resorting to something gimmicky such as Scarf Weavile; you resorted to your Deoxys-S using Taunt and praying that yours went before your opponent's Deoxys-S. This created a negative impact because Deoxys-S created a "stale" metagame. Leads were basically either Deoxys-S or gimmick Scarf user trying to stop Deoxys-S. It created centralization just for a rather simple position. Lead Deoxys-S had no balance like Lead Aerodactyl and Metagross does. Lead Deoxys-S did not create a "fun" metagame either unless you were the lucky one that pulled off the Taunt, Dual Screened, then went straight to your next Pokemon and humiliated your opponent.
In a direct comparison between Deoxys-S's positives versus its negatives, one can
likely conclude that Deoxys-S was a bad influence for the OU metagame. Its negatives to the metagame far exceeded its positives, and thus it was reigned Uber. In fact, Stage 4-1 it did not last throughout the entire testing process before it was removed.
You can draw this with any Pokemon. Scizor still does create a stale metagame within itself. It also does create a negative impact due to taking out viability of many Pokemon within the process. Though, the question still stands if its negatives outweigh the positives? The answer is likely no. Scizor is a person's teammate to likely prevent things such as Gengar and Dragon Dance Salamence from wrecking their team. Scizor, unlike other Pokemon, even have "counters", which while counters being a flimsy argument, it had more concrete counters than Salamence does. It's low Speed also gets in its way despite Bullet Punch. Many people will agree with you; the metagame is Speed dominated. Stall is still around, but it is not 100% viable when you consider the many sleeves that offensive teams can pull to make stall useless.
There is also the Offensive, Defensive, and Support Characteristics that were used to help determine how a Pokemon is Uber. You can slap Lucario, Scizor, Heatran, Rotom-A, and other Pokemon and draw conclusions that it could weigh either way. When you factor Salamence, it becomes a much different issue. Salamence has no counters. For fucks sake, this thing barely has
checks, since the Counter analogy has been dead since DP's existence. It creates a stale metagame because it relies on Scarf Pokemon with base 100 Speed or more to be used to take on Salamence after a Dragon Dance, and the base 100 Speed Pokemon risk a 50% chance of death in some scenarios. Nothing safely switches into it. Yes, you can also draw the same conclusions with Dragonite, a Pokemon that is very similar to Salamence. It's base Speed is its major drawback. It doesn't have Intimidate to help it set up. This isn't saying that Dragonite is powerful, but it can be drawn to be less Uber than Salamence to some extents.
For your tl;dr version: the main point is what you want to see in a OU Metagame. Once you conclude that (which we have, for the most part), we can start figuring out how Salamence causes a negative impact to the metagame. We can logically conclude many of its reasons. We just to make sure that the argument does not make a backfire on a Pokemon that is either clearly OU or at least isn't justified as an "Uber" Pokemon.
The one thing we must make sure is to not use a flimsy argument like an unrealistic set that happens. 5 Moves, etc. Sets such as Chain Chomp were used against Garchomp, which I find to be a very ineffective set and a waste of Garchomp's overall potential.
P.S. I also seek to make Salamence Uber without resorting to a "slippery slope" theory. I think most of us seek that, but there are others that simply don't care and will go and attempt to ban everything they hate (i.e. Scizor) without a valid argument behind it.