For example, Android phones (still!!!) don't support stateful DHCPv6. Moreover, DHCPv6 was designed by idiots and out of many options for DUIDs it doesn't allow the most logical one: a user-specified host name.
PMTU in IPv6 is even _more_ broken than in V4 because extension headers just plain don't work in the wide Internet.
Thank you for mentioning extension headers. IMHO, they are the worst mistake in IPv6, because they are unbounded. If you design your hardware to handle extension headers, there are all kinds of edge cases to worry about, and regression testing the different combination of headers is nearly impossible. So lots of hardware vendors just don't handle them (or handle just one), which means they might as well not exist.
IPv6 still causes troubles in deployment.
For example, Android phones (still!!!) don't support stateful DHCPv6. Moreover, DHCPv6 was designed by idiots and out of many options for DUIDs it doesn't allow the most logical one: a user-specified host name.
PMTU in IPv6 is even _more_ broken than in V4 because extension headers just plain don't work in the wide Internet.