Alluka/Nanika is a crazy asspull and she fucks up the entire consistency and balance of the power system.
That being said, narratively she was badly needed as a character. Killua and Gon's relationship was way too intense and codependent at that moment, introducing a new character that could alleviate it by making Killua had someone else to care about was a relief.
So I'm not even mad about it.