Gen III
>+Gardevoir released
>+Decent stats
>-Out-classed in every way by other psychic sweepers

Gen IV
>+Gains access to wide support move pool
>-Out-classed in every way by other supports
>-Gallade released, essentially a better and more versatile version of Gardevoir

Gen VI
+Gains new Fairy typing
+Gains safe switch in on Outrage spam
+No longer weak to Dark
+4x Fighting resistant
+Gains mega stone
+Gains pixilate, making the threat of a hyper voice Gardevoir significant
-Mega Evolution raised its Attack stat slightly instead of something useful like Defense or HP
-Now one-shot by Bullet Punch (lol lucario/scizor)
-Now one-shot by pretty much any steel or poison attack in the game
-Majority of its best support moves are move tutor, X & Y has no move tutor
-Still out-classed even by other fairies at support anyway
-Hyper Voice move-tutor only, perhaps the cruelest joke of all for this unfortunate 'mon

Gardevoir: The forever almost-good pokemon.