Gardevoir is a normal pokemon, it isnt too powerful, it has a powerful mega evolution but thats it, it doesnt have a lot of, health, or defense and its speed is meh, it has clear weaknesses and strengths, i find it kind of hilarious how Alakazam was available to get(in a previous version) but not Gardevoir, anyhow, Eevee has some good eeveelutions like Vaporeon and Sylveon, but its best eeveelutions are just as strong as Gardevoir.
I used Gardevoir during Pokemon Reborn and the performance of the Pokemon was normal, it didnt feel overpowered.
Vivillon is better than Gardevoir as a sweeper(without a mega evolution) during the late game because it has access to compound-eyes boosted Sleep Powder and Hurricane, along with Quiver Dance and a bug STAB, i am planning to catch one and be patient with it, its not good in competitive play because of its typing but its a different story in Single Player.
Some available pokemon are even better than those two are at what they bring to the table(such as Chandelure).
Anyway i dont want to start bad blood, everyones entitled to their own opinion and ill respect every one.