It's not the most powerful, it's the most illegal. It doesn't get used at all when Dumbledore and Voldemort duel, because they're both too powerful to lose to it.
What am I doing defending these books? I hate them! Alright, lemme mention that Cho Chang is a racist nonsense name to balance my defence out.
At least those are two Irish names. Cho Chang is a nonsense name. It's like calling a European character "Jackson Esparza". Bauer O'Brien. Ferdinand Thörson.