The benefit to necro/sonic is that, after the level 32 upgrade, your pets have a self-heal, extending their time before catastrophic failure. It’s not perfect, and positioning is still critical as @Demon Shell says above. With the +res procs added in (+35 max), the individual shields, and the resistance bubble, your pets are quite hardy (when -hit and +def procs are added in). Getting the mastermind’s defense at least within one defensive skittle (32.5), a necro/sonic is quite tanky with the help of bodyguard mode. You’re own healing can be addressed with either Life Drain or the medicine pool.
In addition, the grave knights are excellent anchors for both Disruption Field and Sonic Repulsion (with the KD proc). With the Overwhelming Force proc in the zombies, it’s like 4 marauding bowling balls in a mass of pins. Make a circle around your victims. Stand at the edge of Supremacy range, the Lich 90 degrees around the circumference to catch the spawn in its cones, and send the rest into the scrum. Most of the time, it works!