SLAAC is a huge and complex part of IPv6. Higher reliance on ICMPv6 is also a big part of it. Networking stacks for IPv6 are also more complex, especially if you want to support SLAAC, requiring things like multiple IPs on every machine by default, and so on. The very fact that you have to choose between static IP, SLAAC, and DHCPv6 is another complication - if the choice is even there, as some major devices don't support DHCPv6 (Android).