IPv6 is sound in most of its specifications, implementations, and operations, but a few shortcomings can make it bad for users. For example, if a user starts using IPv6 on a trial basis and is significantly less comfortable due to minor shortcomings than using IPv4, he or she will revert to IPv4.
This is actually happening because of a few shortcomings. Therefore, the WIDE project has begun activities to correct the shortcomings of the IPv6 specification, implementation, and operation. The name of the subcommittee dealing with this is IPv6 Fix (v6fix).