Snatch requires you to both be at the right place and time. I mostly use it on leads in Battle Spot Singles so that I can Snatch a Shell Smash from Cloyster. :P
Trace is very unreliable. Even if you nab an Immunity, it usually won't save Gardevoir (or Alakazam) for that matter. :P
Tapu Lele's Psychic Surge boosts Psychic moves by 1.3 and makes it immune to Priority moves, which stops it from being Checked/Countered by Mega Scizor and Mega Mawile. Gardevoir is a little less frail than Mega Alakazam (Mega Gardevoir has 68 HP, 65 Def, 135 Special Defense, Mega Alakazam has 55 Hp, 65 Def, 105 SpD), but still often goes down in one hit and is slower than Tapu Lele and Alakazam, which makes it a less desirable Calm Mind user, while Tapu Lele can take a hit from more Pokémon and secures more KOs with more Speed. Mega Gardevoir is only better than Tapu Lele in Doubles, otherwise, you should Probably use Tapu Lele or Mega Alakazam. :P