Keep in mind this is an anti-meta deck, so it's going to struggle against anything that it's not specifically tailored to beat, so, you are correct on that point. Latias IS really good, but more other things than Trevenant, since that deck plays Espeon-EX now. Even though it's a 3HKO, Trev will literally never let Latias KO Espeon with Flare Grunt and 4 E Hammers. I think this deck is good against what it beats, but loses to everything else. That's usually why anti-meta decks never win events. Another note on Latias is that it can't 2HKO Trev because it disables Lasers, Muscle Bands, and prevents you from playing Special Charge or Puzzles. And if you try selling with Latias and you're forced to bench another Pokemon to prevent yourself from losing the next turn, Trevenant has another target to take prizes from.Playing against deck that does not play special energy seems hard. Darkrai and Volcanion decks come to mind. Getting DDE and DCE on the second turn seems hard to achieve in a consistent manner. I mean i could still be wrong but it doesn't seems clutch to me.
Regarding Trevenant, i guess you could easily play Latias-Ex in the deck. He is an auto-win in most cases granted they do not play uncommon attackers or Silent Lab.