I've always found the most complicated part of IPv6 to be address scopes and source address selection. The fact that one interface can have any number of addresses in different scopes and prefixes complicates things a lot.

Another thing that will always trip up new IPv6 network engineers is solicited-node multicast. You know the theory, computers talk to ff02::1 for neighbor discovery and then you hop onto a real network and see none of that actually happening.

And probably the most complicated thing for network engineers - how to set up firewall rules if machines are constantly changing their addresses.

For developers and security people - just parsing and validating v6 addresses is a whole bunch more work, but at least for this, the tools are available to help you now.