Not everything, for example if you're slaughtering thralls you're going to want a mythoclast and if there's lots of vandals you're still better off using Hawkmoon. But when it comes to boss fights, the wolfpack rounds slaughter everything, and that's when there's only one Gjallarhorn. Two Gjallarhorns takes down Crota's shield instantly, 3 or more and we're talking killing Atheon within one time's vengeance.