Comparing CKN with longbow is like comparing Impis to Samurai. Longbows are better than CKNs in small groups because they take less damage but if you are able to spam CKNs they do a lot of damage within a very short time. Therefore CKNs are arguable better in the offense because they can finish the job faster before the opponent can respond with higher era units and defensive buildings.
I don't think I agree with the "Longbows are better than CKNs in small groups" comment. Try a thought experiment with 100 LBS vs 100CKNs on plains, and you will see that LBs will win. However, 1LB vs 1CKN favours the CKN slightly.