>If all the vendors would apply the same transition mechanism, that would make it a lot more easier for carries to deploy IPv6.
The nice thing about NAT64/DNS64 is that there is no support for anything beyond IPv6 required on the client - it's entirely handled by the network, and is completely transparent / invisible to the endpoints.
464XLAT (https://tools.ietf.org/html/rfc6877), is a stateless NAT46 on the phone, coupled with a stateful NAT64 on the headend, and sometimes with DNS64.
This (edit: 'this' being the presence of stateless translation on the device) means the device has to do extra translation work on a per-packet basis, which would not help the battery life, and also the apps remains IPv4-only. I.e.: it is technical debt.
Apple has a stronger influence on the developers + a lot of address-family independent APIs, so they could exercise this option of allowing operation on IPv6-only access networks while also progressing towards all applications being future-proof.
If you tunnel the DNS resposes over a TLS connection, then it would be secure. This has been specified in an RFC recently.
And about XLAT464: Apple could've just added that to iOS, instead of this. Google did the same a few years ago. If all the vendors would apply the same transition mechanism, that would make it a lot more easier for carries to deploy IPv6.