I think he's specifically talking about the ra-den weak aura. On mythic, there's an extra orb, meaning you have to deal with two "unstable" mechanics at once. On heroic, you can just set 6 players in an order for Vita in case one of your 5 is targeted by it, but in mythic the extra variable of two people potentially being targeted can really screw with your order. The weakaura detects who has vita or the new debuff, nightmare, and alters assignments accordingly.