Yeah Prep KBE is only guaranteed after a successful Prep 4 aGI which is risky enough with the narrow window lol. Not to mention that the move can glitch and make Raph spin 360 and kick the wrong way. Furthermore, one you Prep 4 successfully, the massive pushback after Prep K can make the second hit whiff. It's Raph's least effective BE though it has RO, W!, and big damage potential.