There was a pretty good reason that people stopped playing it, and as a very avid Gardevoir player I can tell you why - you just had too many poor match-ups. Your only great match-ups were against Yveltal decks that didn’t run Garbodor, if they did run Garbodor it was just a good match-up for you, and then you had poor match-ups vs the toad, Landorus-bats (if they didn’t whiff then you’d never ever have an Aromatisse), Donphan (while it was still popular) and Night March, which is still an awful match-up for this deck. Basically anything speedy and/or disruptive would just give you hell.
The deck itself worked wonders but the meta just wasn’t right for it, it was good at taking out other mega Pokemon and the only one that saw any play at the time was Manectric.I stopped playing just after ROS released so I’m not sure of how the situation was after that but during the PRC era I’d say that Gardevoir-EX was more of a fun deck for fairy enthusiasts and nothing more.
It’s interesting to consider which version of the deck would work better - the one with Giratina and DDE or one with Regirock and Exp. Share. In the current meta-game I think both have their merits but quite frankly my experience on ladder so far is that Vespiquen and Night March just makes it so difficult to set-up by 1HKOing your Xerneas starting on turn 2 that it’s annoying.