lol garchomp is uber (wait wasnt her put back into OU o.o, well for point of this thread ill say why he was put into uber) because he has very high stats, and with sand veil it increases its evasion by 20%. this might not seem much but:
on earthquake accuracy becomes 80%, this is 1 in 5 miss.
on hypnosis its 60x0.8 = 48% accuracy, hypnosis now misses more than half of the time.
this mixed in with its high attack stat, respectable speed stat, and wide movepool, makes garchomp a very nasty threat.
common chomp:
choice band
adamant
outrage
252 attack evs
this can OHKO most switch ins, and the ones it doesnt, it 2HKO's instead, including the oh so popular skarmory.
oh the remaining note, you still get common threats such as weavile switching in, 1 stabbed ice beam from weavile will kill chomp, unless its holding focus sash in which case weavile will get OHKO'ed itself, or even if it doesnt, because of sand veil there is always that chance of ice beam missing to start with.
unsuprisingly, lots of people use sand veil to thier advantage, but generally not with sandstorm, lots of people go with T-TAR leads for permenant sand storm