SLAAC was just an example, but even here you're downplaying. SLAAC adds a lot of extra complexity: every host now has multiple IP addresses, which was a rarity in IPv4 land, especially for client machines in a LAN; and exactly which IP should be used to identify a host is now highly dependent on details of the network config that are hard to guess (is it using privacy extensions? Will this IP expire daily, or is it more stable?). Plus the dependence on ISPs assigning more than a /64, otherwise SLAAC is broken.

And there are also other things - more dependence on ICMP, the concept that your LAN IPs are determined by your ISP (unless you also assign ULAs in addition to all the other IPs), and probably others I'm forgetting.