Abomasnow has risen to OU status. What more are you asking for? For him to be in the top 10?
Sand Veil is not "free", it takes setup time just like any other weather-based ability. In essence, it takes 2 pokemon to counter Garchomp when Sand Veil is up because you are in reality... playing against 2 pokemon. IE: Garchomp + Tyranitar, or Garchomp + Hippowdon.
No respectable Garchomp set would have Sandstorm on himself. Therefore, it must be supported by other pokemon. With equal amounts of set-up (You're giving Garchomp a Sub + Sandstorm), Walrein is an especially difficult force to be reckoned with (Sub + Hailstorm). No set completely counters a Stallrein because of his flexible movepool. T-Tars die to Surf, Gyarados dies to Blizzard / Rock Slide, Metagross is Sub/Protected to death (Surf kills him before he kills you with Meteor Smash anyway, esp. with a sub up), Tentacruels are Earthquaked. Blissey, Cresselia, Skarmory and etc. etc. are easily roared away or face a Toxic.
And all of those are using the cannonical Sub/Protect/Roar set, Yawn, Encore, Curse, Stockpile and others can completely screw the expected strategy. (Encore pokemon comes into a Yawn for example instead of a Sub)
My point: If you let someone set up to that point: of course they deserve a massive advantage. And if you just assume that something is set-up already, that is a bias towards Garchomp. I can admit that a SD Garchomp is probably a bit more threatening than a Walrein behind a sub, but I've won entire games stalling people out to a Stallrein all alone in a hail team as well. My core point is again, it is unfair to consider Sandstorm to be "free" when considering Garchomp