One is that I'm just more used to Garchomp, only having gotten a well IVed Latios recently. Garchomp's immunity to Thunder Wave is also very helpful. But the biggest thing is that though Latios' special attack stat suggests he hits just as hard, he's using weaker attacks. Choice Band STABed Outrage and Earthquake 1HKO much more than Dragon Pulse and Psyshock or Psychic, which really makes a big difference. I tried Jumpman's pure attacker Life Orb Latios set, which has amazing coverage with Thunderbolt and Ice Beam as non-STABed attacks, but still found myself unsatisfied with Latios's raw KO power. I could use Draco Meteor, but I've gotten into trouble too many times with untimely misses so that 90% accuracy is a turn-off.
I do agree, however, that a Memento set has promise, but I have not yet used it in the Subway. I'd love to try out a hybrid hit hard-then-Memento set at some point in the future.