I was told by mathletes that KC is worthless on anything but trash mobs because
1) You lose a crapton of DOT damage from the poor DMG on it.
2) You feed tons of TP
3) The distribution of multi-hits doesn't put it ahead of something like a STR magian club.
The strongest mob you're going to be using Melee WHM on are like ... normal mobs in an Einherjar type event or Limbus type event, and any WHM (even melee WHM) worth bringing will drop back to the backline for any boss type monster.
Our calculations for Mjollnir (specifically Austar and Motenten's, I haven't really posted mine) were using Dynamis DC as a simulation for WHM-melee friendly targets, and both theirs and mine put KC strongly ahead of other options.
The main difference I've seen is that I get Realmrazer to be further ahead of Hexa Strike than Motenten did, but it's possible I just used a better gear set or slightly different conditions since I had to add some pieces into the spreadsheet myself.
There's really no point in considering WHM melee as a viable option versus anything extremely difficult, as you'll want to take advantage of the safety of the backlines.