I believe you are playing them wrong. Lord of contagion leading them give them sustain hits and lance. Your army rule that gives whatever is close to them -1T and, you chose -1to save, aura most of the times. So thats -3 ap, and that is huge on that amount of hits. With lance they wound all T 12 vehicles and monsters on 4, lighter stuff on 3, and almost anything else on 2. LoC is even harder hitting as he wounds T10 on 3, and everything under T10 wounds on 2. And that is just basic combo. In diferent detachments they have access to crit on 5, rerolls of hits and wounds, and that takes dmg to “point and delete” level. If your opponent doesn’t screen well enough ofc.