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There's a chance as many mentioned before that Killua lied about Nanika's healing ability having no backlashes, i think it might be different when it comes to healing like you set the price upfront before wishing someone be healed(like Killua sacrificing his friendship with Gon), if no price is set Nanika/Alluka herself pays it by sleeping or so.
If the backlash endangers tens of thousands of people, no doubt Killua doesn't want Gon to know that