In terms of potential, and what it's competitors are doing? Yeah, it's been a huge disappointment.
Killzone 2 was the peak. Killzone 3 changed too many core gameplay mechanics. I don't think I've ever been let down with a game more than that one there.
Shadow Fall is more like Killzone 2 than 3, but they botched the launch. I honestly think they should have waited to drop this game against Titanfall. Anytime a multiplayer game launches without a beta... bad shyt will happen. There's no way in hell Guerrilla would have been able to release the game without mic support for random players to communicate. Player feedback for a lot of things for the game would have greatly improved the experience had they had a beta for the game.
It's still a decent game, but they picked a difficult time to drop it. It sold well... but I think the game really could have benefited from a beta and some more time to make sure everything just made sense.