It was like Jotaro getting timestop at the end of Part 3. An asspull, but the author managed to do something interesting with it, and at least it was moderate (no Nen back for Gon, no instant win for Jotaro like with Giorno). It doesn't even bother me that Alluka gave Killua a wish for free, if it only can give "clean" wishes without consequences it kinda fits with HxH's general theme of human being bastards (the calamity isn't bad by itself, it's made bad because humans' greed). In a reversal of his usual cynicism, the assassin clan that studies Alluka in a cold, scientific and dehumanizing way, couldn't figure half of what Killua learnt by treating Alluka and Nanika as human beings.
I really loved the family brawl and the conflicting motivations that ran even among the servants. Even if you have a magical key to fix everything, you can create conflict making the characters disagree about how to better use it.
There's also a chance that Killua might have been hiding more rules, maybe a selfless wish implies some strict rules (Killua got a free healing because he honestly wished his friend to get better, you can't for example wish for an army to revive just to send them to fight again).
And both with Jojo and with this, if you make an effort, you can make it make sense, but it's clear the author didn't have it well thought (in Togashi's case, he left a clue with the Zoldyck kids' names, but for me he just wanted to leave open the possibility of add another Zoldyck).
The point is, maybe it was an asspull, but that doesn't make it neccessarily bad.