Phoenix, Breach and Reyna's flashes all have an audio queue before their flash goes off so you have a short window to react to it. Omen's flash doesn't have an audio queue but the edges of your screen become purple so you can still prepare for it.

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But Skye's flash makes 0 sound while flying.

Not only does she have 3 flashes, they can also curve, be controlled, timed and even make a noise to tell you if an enemy got flashed.

Now i'm not saying you should butcher her with massive nerfs but i think her E: Guiding Light should at least make a noise as it's flying around otherwise it's to hard to do anything against.