I'd argue that it's still not mature. ULA/GUA mess is one example, I recommend reading this delightful thread with beer and popcorn: https://mailarchive.ietf.org/arch/msg/v6ops/7-SuigPyP0R-C6ZC...

Having to waste 64 bits for the local part of the address is another one. There's an RFC in progress to allow neighbor discovery with shorter prefixes, so we might be able to use it by 2045.

And Android _still_ doesn't support DHCPv6.