LOL...

A projectile (kunai) thrown by the arm has less force upon impact than a punch. Especially if the kunai is travelling a considerable distance.

I don't see how you can simplify Gaara's ability to that level.

Obviously, you always try to stop a strong character before they use there strongest jutsu.

For example, you want to kill Itachi before he uses mange. Same as Gaara, you want to kill him before he turns into a tanuki. Common sense