This is the second upgrade of the Path 1 for the Ninja Monkey. It costs $300 (Easy), $350 (Medium), $380 (Hard), $420 (Impoppable). Shurikens can pop 4 bloons each and is especially useful if combined with Seeking Shuriken. This allows the Ninja Monkey to deal with crowds twice as well. However, this should not be used as crowd control as it still is not enough to pop bloon rushes.
A ninja monkey with sharp shurikens can be used for early game because of its high pierce, fast attack speed, low cost, and ability to pop camo bloons.
Sharp shurikens aren't sharp enough to pop leads or iced bloons.
It is better to crosspath Ninja Monkey with only up to Ninja Discipline for most occasions, especially for the Upper Path 2 upgrades for BTD5. The only time Sharp Shurikens will come in handy is when extra shuriken pierce is required, which would become near-negligible outside of early-game.