Just like izanagi, Grand Overture had a nerf where you were able to stack your barrage on objects that said immune. Although it was a glitch, it made the weapon more fun and useful. The weapon also got nerfed right after its' performance on day on e Vesper's Host

Ooooh yeah, that does suck especially since the new primary gl can still stack off of immune mobs. Still a very viable gun though.