The answer is simple, it's an annoying way to die. Tripmines and fusions and flux and lightning nades kill too but it's not as annoying. Same reason people call nerf in firebolts, it's a slow, painful death.
I think it's fine because at baseline the nightstalker nades are horrible.
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I can agree with that, annoying but still not OP, and definitely not deserving of a nerf. I feel it makes a good player to know their resources and use them to their advantage.