Your logic is flawed once again. If no one uses sleeper in crucible and suddenly everyone is using it in gambit. That tells you that the gun is not just considered a generally good gun, but is instead clearly too powerful in gambit. Which is exactly what this post relates too, gambit.
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What do you get in gambit that you don’t in crucible... seeing through walls. Guns with a charge time and the ability to see through walls is an absolutely overpowered combo.