Comment by positr0n

I think the people complaining about compatibility are more talking about the concepts in IPv4 and IPv6. IPv6 could have been "everything is the same except the IP address is 16 bytes instead of 4". Instead there are new ways to do everything.

Addressing works differently (no broadcast, multicast everywhere, link-local is mandatory). Configuration works differently (SLAAC, RA, DHCPv6 is not a drop-in replacement for regular DHCP). Neighbor discovery replaces ARP and depends on ICMPv6 working. Fragmentation behavior changed. NAT is “not a thing” by design, which breaks a bunch of assumptions people built entire networks around.