https://cr.yp.to/djbdns/ipv6mess.html
still as relevant as the day it was written
replies(3):
That’s like saying HTTP can talk to FTP servers as long as there is an HTTP to FTP proxy.
The only thing that makes them seem compatible is there is a well formed address space in v6 that clients send v4 requests to. But it’s still v6 and a 64 proxy needs to have an actual IPv4 address to translate the source to before sending it via v4 to the actual destination.
Which was true of all the IPng candidates, and not just the one that ended up being chosen for "IPv6".
There is no way to expand the addresses space (as found in IPv4) to something greater that 32-bits in a compatible: new API calls, data structures, DNS records, etc, were always going to be needed.
To list "not compatible" as a con of IPng/IPv4 is non-sensical.