I had been(hopefully past-tense!) struggling with mealies on one of my huernia for a long time. Everything I tried worked more or less, but they'd be back in a month or so. About a week ago I unpotted it and, sure enough, they were on the roots. A huernia doesn't mind drying out, so I treated the roots, let it be unpotted for several days, picked everything buggish off with a toothpick, retreated it(now in several sections, but that's ok) and potted it with fresh soil in a fresh pot yesterday. If you can't get rid of yours, or if you do but they come back, suspect and check the roots!