It's not that is doesn't work, it's just a very situational thing. Let me try to explain it. -When aiming down the sights, you are stationary/moving very slow. If enemies are attacking you during this time, they will be attacking at the last known location of yourself. -This location will generally be at or near you, depending on how much you are maneuvering while ADS. -This dilemma renders the invisibility almost useless, unless you are outside their field of vision, turn invisible, and enter their vision radius in which they would see non-invisible Guardians.