Gjallarhorn was one of *maybe* two exotics nerfed for PvE reasons over PvP. Nearly every other nerf since launch has been due to complaints in the Crucible.

it was because it could potentially kill the entire team if they were close together. as originally fire it into a group traveling together and even if the main explosion didn't kill you the (agressive tracking) wolfpack rounds did almost enough to kill a player with a single round (and it released around 10 traking rounds) without the main rockets explosion in PvP so it was nerfed to release less wolfpack rounds and reduced damage from the wolfpack rounds.

No, it's wasn't... Here are some things that were. Titan Hammers (Ruined) Warlock Stormcaller (Weakened) Warlock Sunsinger (Weakened) Hand Cannons (Ruined) Fusion Rifles (Ruined, fixed, Ruined, Fixed) Shotguns (Ruined) Pulse Rifles (Weakened) Machine Guns (Ammo Nerfed to Uselessness) All sorts of exotics (Weakened, Ruined, Nerfed) And, my favorite... Ammo Capacity.