To me, it seems like luckamancers aren't any better or worse off relative to other casters.
From Clay's perspective, it may seem like they are stealing luck from other units on their side, but it really comes down to a choice about which units to boost and which not to. Every caster, or their commanding warlord or ruler, will have to make that choice, and some units will benefit from it while others will go without.
Healomancers can't heal everyone on the side each turn (can they?) so their placement and usage will be the same type of choice, with the same types of results. Units in range get heals, other units don't. But healomancers don't seem to view themselves as stealing hits from those other units. But the effect is the same.
Clay stated that he must be stealing numbers from his own side because otherwise mathamancy would be off, but wouldn't mathamancy account for a luckamancer's effect on the sides involved? He also said they lost more unled battles, but that could just be coincidence, or maybe possibly the ginormous side they were fighting combined with wavering allies leaking intel.