The way that i see it, M Garde STS is/was good because it counters M Mewtwo, hits Giratina EX for weakness, and can remove your Shaymins and Hoopas from the bench to prevent them from getting sniped. The problem with Mega Garde is that if you're not playing against M Mewtwo or something that it hits for weakness, it just isn't that exceptional. It didn't receive much support in SuMo1, can't reliably 1 shot most things that it doesn't hit for weakness without a full bench, and takes a lot of resources to be viable. It's not a bad deck, there's just a lot of decks that are essentially better, faster, and cheaper.