KUS is considered one of the toughest FPs... again. 10 minutes enrages are tough to beat, yeah. And bosses there have so much tactics and require careful planning. You must know exactly what part of your face you will roll on your keyboard.
Kaon's bosses are faceroll, sure, but it's clearly more rigorous than T5/MP/FE/Esseles. You could make the case that it's easier than BoI, I suppose, but imho the trash requires a fair bit more effort, hence my ordering.
Another way of looking at it is that KuS is the instance most likely to wipe my group if we're too busy laughing to play properly.