Most of my khukuris - 7 of them - I really do use.
Some I use more than others of course.
I have noticed with the kardas that some are harder than others and I can tell by the sound they make that they do seem to be zone hardened because only a specific area of the edge *rings* when I steel it.
But some of them are significantly harder than others.
The hardening of the kardas which came with all my chitlangis seems to be reasonably good. I've used the long karda which came with my 26" Chitlangi (Bura) to shave with and it worked fairly well.
The karda from my 20" Ang Khola (Kumar) is so-so but serviceable.
The karda from my 25" Sirupati (Vim) is very very good. of all my kardas, that one has the hardest edge. It took nearly an hour with my diamond hone just to get the edge profile where I wanted it.
My most recent H.I. acquisition is an odd one. It's a 20.25" Sirupati by Bura which came with two kardas instead of the usual karda-chakma combo. One of them has a very hard edge which is hard to about the last 1/8" at the tip and the other is barely hardened at all. They also arrived with the usual high polish blades but the handles were kinda rough; unstained, unpolished, and having more of a Villager appearance. I was able to easily remedy that though so it's not a big deal.