In my testing, I've found that Garchomp/Altaria is really just a hybrid of Reshiphlosion and Machamp. Let me explain why.
Reshiphlosion was a really consistent deck, and spewed out high amounts of damage, consistently, every turn. The problem with Reshiphlosion is that it presented no ability to really outplay your opponent (not necessarily a bad thing, given the positives of the deck), and that it was literally on a rail-track. It was really good at what it was supposed to do, but it didn't possess the ability to do anything else. Granted, that's fine, because it was the BDIF for a bit, and it was a very, very solid option if you wanted a consistent deck choice at it's high point, but it really wasn't as skill intensive as say, The Truth.
Machamp, from two formats ago, worked in a similar fashion, in that it just plowed through SP decks (and, with the prime, did big damage). It was fast and had great early game power. The problem with Machamp was that it was completely an auto-pilot deck, and did absolutely nothing for players who wanted the skill based game of Chess that a Luxchomp mirror provided. It was consistent, which is why it saw play, but it suffered from the same problems Reshiphlosion would have (they both had similar positives, however).
Garchomp/Altaria is an exact mix of the two. Garchomp/Altaria hits for very high amounts of damage, very quickly, and very consistently throughout the entire game. Garchomp/Altaria, however, lacks a lot of the higher level plays that decks like Eels or Hydreigon/Darkrai can provide. That's not to say it's a bad thing, necessarily, because the deck is consistent and possesses the ability to win, but it's just kind of the..."lame" deck of the format. It can't really do anything besides catcher up the next target and hit it for huge amounts of damage. All this jazz about Altaria being really weak and whatever are all warranted, but the it does provide Garchomp with the potential to two shot everything in the game. Garchomp itself probably won't get KO'd either, so even if Altarias are getting picked off, Garchomp can still handle a lot on it's own with the second attack. I put the deck at like...tier 1.5, because it is still really consistent and a decent play.