Salamence wishes it had Ground/Dragon STAB. Earthquake and Dragon Claw are all it takes for Garchomp to rip through teams. It doesn't have to resort to running mixed or gimmick sets.
Garchomp will always take down at least one key player in your team and there is nothing you can do about it.
A common scenario would be Garchomp comes out against your Heatran locked into Fire Blast, you switch in Latias as Garchomp uses Sword Dance. Then lets say you attack with Dragon Pulse activating it's Haban Berry or even worse you miss because of Sand Veil. Now you have just lost what may have been your only pokemon capable of revenging Garchomp and now you're at risk of losing the game. Now sure you can apply this to many of the OU sweepers but Garchomp is unique in that your 2nd attempt to take him out can be nullified by Sand Veil.
Highest ranking on OU ladder: #4
Highest ranking on UU ladder: #1
Highest ranking on Uber ladder: #19