Dear Masataka,
my interest in this is national security. I see a problem with IPv6 in two areas.
Only two?
IPv6 contains a lot of unnecessary features, such as stateless autoconfiguration, and is too complex to be deployable.
Comments welcome.
As it is too complex, it naturally has a lot of security problems.
I'm not surprised some of them are national ones.
Masataka Ohta