@phoenix_fire:
Why going so in depths with these rules in multiple chapters, if you discard it all in the end and let Killua use her ability for free?
1. The rules were complex with caveats that were constantly be explored and revealed, and a large part of the trick was only Killua knew one of the sneaky bit. No different than many, many other nen abilities.
2. Only for healing, and only face to face, not for anything else and not from a distance.
3. He still doesn't get it for free. Still had to get his sister on site, still had to hide the truth from his deadly family, still had to fight his family,is now targeted quite seriously by said family, still lost his Butler, and now has to separate from Gon for an indeterminate amount of time and go into hiding and protecting. There were costs involved in getting to the wish and from having used it… just not from the wish itself.
If he'd made a freebie wish to circumvent those difficulties, the actual heal wouldn't necessarily have worked because the inner monster might have been mad at him for the abuse.
Yes its a bit of a miracle cure, but that's kind of the point. It really is no more cheap than restoring Gon's arm after Greed Island. That really WAS a freebie, it wasn't even within the special end-game wishes.