That is probably the best scene in the game.
It doesn't really make sense that something that can cut through Nethicite, which nothing else can, can not chop someone in half with equal ease.
It is a let down, and it would seem that the developers didn't think it through. Just like when they thought that having 10 hours between any story would be a good idea (Sandsea/Trip to Archades/etc). Or that Vaan should be the main character.
Let me stop before this becomes a full on rant about XII.