Sneak and you gain Speed III and Jump II for 5 seconds. For that whole time, your attacks are cancelled — sword and bow alike.
That is what stops the Karasu being a speed kit in disguise. Charging your target is pointless: you arrive unarmed, and you have to wait out the flight before landing a first hit. The ability exists to leave a position, never to take one.
The jump matters as much as the speed: without it you hit the first wall and get caught. With it, the terrain opens up.
The counter is simple: follow him. The crow does not turn invisible and does not go through walls — he runs fast, unarmed, for five seconds.