Essentially the old "breaks the setting/renders the story moot argument" which is an incredibly half-assed argument at the best of times, but even more hilarious in situations like FFVII where they miss the point entirely.
Sephiroth's plan was to WOUND the planet, Supernova (which was already a move he had as far back as Crisis Core) would have DESTROYED the planet. Which is why he needed the Black Materia, to use an attack powerful enough to wound, but not kill the planet essentially.
The planet (which was able to stop METEOR) was held back from interfering specifically by Sephiroth. Now remind me, who defeated Sephiroth again? Yet these assholes are acting like METEOR is above the party who beat the guy who held back the planet from stopping METEOR in the first place.
Next they'll try and say Vegnagun being Sin level "breaks the story" of that game. It doesn't, it just creates a power creep that has the goofy ass Leblanc syndicate not dying instantly against a Sin level opponent.