Yah OP your right, logically in almost any one on one fight. Kakashi should be able to Kamui he's enemy. Kishi wrote himself in the corner with that technique. Before it took Kakashi a while to use it, but now he's able to use it on people at the same level as Naruto's speed (sucking his clone up vs Obito).
But Kishi is going to ignore that fact, because he can't let Kakashi solo everyone.
This is manga is great, but Kishi does ignore a lot of physics or fact in places into the manga. But what can you expect from a manga that when a person dies, most likely its a clone lol.