I'm unsure whether or not you're talking about Mega Gardevoir or normal Gardevoir. I'll assume the former.
Steel-types: though it often carries Will-O-Wisp and Focus Blast, a lot of good Steel-types can counter it should they switch in freely, like Mega Scizor, Mega Metagross, etc.
Faster Poison-types: while Psyshock easily OHKOes the lot of them, faster Poison-type Pokemon like Mega Beedrill & Gengar force it out easily and can inflict massive damage on the switch-in. Mega Beedrill, in particular, can collect momentum with U-turn upon a forced switch.
Faster physical attackers: as its Physical Defense is so low, many faster physical attackers can easily bring it down. Mega Gardevoir variants lacking Will-O-Wisp are countered by bulkier, slower attackers, too, that can take a hit from it.
Hope I helped!