IPv6 is a recursive WTF. It might _look_ like a conservative expansion of IPv4, but it's really not. A lot of operational experience and practices from IPv4 don't apply to IPv6.

For example, in IPv4 each host has one local net address, and the gateway uses NAT to let it speak with the Internet. Simple and clean.

In IPv6 each host has multiple global addresses. But if your global connection goes down, these addresses are supposed to be withdrawn. So your hosts can end up with _no_ addresses. ULA was invented to solve this, but the source selection rules are STILL being debated: https://www.ietf.org/archive/id/draft-ietf-6man-rfc6724-upda...

Then there's DHCP. With IPv4 the almost-universal DHCP serves as an easy way to do network inspection. With IPv6 there's literally _nothing_ similar. Stateful DHCPv6 is not supported on Android (because its engineers are hell-bent on preventing IPv6). And even when it's supported, the protocol doesn't require clients to identify themselves with a human-readable hostname.

Then there's IP fragmentation and PMTU that are a burning trash fire. Or the IPv6 extension headers. Or....

In short, there are VERY good reasons why IPv6 has been floundering.

> For example, in IPv4 each host has one local net address, and the gateway uses NAT to let it speak with the Internet. Simple and clean.

I assume you mean "interface", not "host". Because it's absolutely not true that a host can only have one "local net address".

EDIT: a brief Google also confirms that a single interface isn't restricted to one address either: sudo ip address add <ip-address>/<prefix-length> dev <interface>

How do the working IPv6 deployments cope with these issues?

The reason: Skill issue.