I had the opposite experience, with even the first three mythic bosses having a lot of nasty raid damage, overlapping mechanics, sudden damage spikes and Harjatan trucking tanks even harder than Kil'Jaeden. Compared to that, KJ is pretty much down to Armageddon soaking - once you get past phase two, it becomes a lot more forgiving. Obelisks hurt, but a single hit doesn't kill you, red ball of fire is easy to avoid and Darkness is just a typical damage aura. Half of your raid can die fairly quickly and you'll still be able to finish him off.