1) it depends on what kind of lice the chickens have- there are several types. ivermectin will kill the kinds that feed on blood, not the kind that chews on feathers. what kind do you see (where are they and what do they look like?)
2) the topical powders (permethrin/pyrethrin & carbaryl) will get both types.
3) ivermectin kills many internal worms, as well as mites.
4) ivermectin is off label for laying hens, so has lots of 'baggage' if you use it.
5) ivermectin can/will kill if you overdose it, it has a much more narrow range of safety than the topical powders.