Who dodges kunai these days?lol. On the serious note samurai swords can cut off heads but as seen in this manga the samurai run their chakra though their blades to make them stronger(most likely due to the fact that the sword would break on samurai armour). While zabuza's sword doesn't need this and most likely will still send the head flying.
They run chakra through their blade in order to combat ninjutsu, that doesn't mean they can't cut through a head without it....that wouldn't make sense
especially since Zabuza's blade is made from iron and not from steel, and you're claiming that it's the only blade without chakra that can decapitate
Itachi's kunai without the use of chakra took off Orochimaru's arm....if only a kunai could do that, then swords can most definitely cut off heads.
what about the Hiramekarei? I'm sure it can easily cleave heads off without the need of chakra as well.
the Kabutowari has no chakra flowing through it, and it will surely decapitate heads easily.