I could argue about why it's so easy to track objects moving in straight predictable paths, vs a ninja retreating from him going in every direction, especially if someone like Deidara was to send exploding clay birdz at him while he was trying, and the trade off is too great, he uses it once or twice and is bed bound for a week, sorry to say but Kamui sux1. yet to kill anyone doesnt mean it cant because we havent seen him use it alot maybe 5 times or less?
2. he can kamui someone's head off? he was able to kamui a a nail going at the speed of a bullet from deva pain, he kamuied a fast moving rocket from asura pain? he kamuied sasuke's sussano arrow?
give kakashi a few more years of using kamui it would be more deadly.