I have CoreELEC 20.3-Nexus on a Minix U22-XJ and noticed quite a few problems with the IPv6 support:
1, IPv6 is not enabled by default. While savvy users can turn it on, many users will not know about it. A lot of ISPs use CGNAT, so not having IPv6 by default results not only in a slower connection and greater likelihood of getting banned or hitting per-ip rate limits for that user, but the extra load on the CGNAT gateways caused by streaming videos can impact other users as well.
2, The zeroconf browser can see IPv6-only hosts and services on the LAN, but is unable to mount them (see attached debug log). For dual stack services it only ever connects via legacy IP which is undesirable.
3, Settings/connections/wired on a dual stack network only shows the legacy address and does not show the IPv6 address, on an IPv6-only network no address is shown at all. Wireless is probably the same.
4, Settings/connections/wired/DNS Servers - the input field only allows legacy IP addresses to be entered manually, but it does display the IPv6 nameservers acquired from DHCPv6/SLAAC when running in dual stack mode.
5, Having an IPv6-only connection results in no DNS servers showing up either through the UI or in the /etc/resolv.conf file, despite the fact that the system uses IPv6 nameservers when in dual stack mode. This is likely at least a partial cause of 6 below.
6, When an IPv6-only configuration is in use, attempting to access any URL fails stating the network is offline, despite this the system is still able to download updates, can still be accessed via SSH and command line tools like wget/curl are able to access the same URLs just fine if you add a nameserver back to /etc/resolv.conf.
7, Manually adding an NFS url to an IPv6-only host fails either by IP or DNS. If a dual stack host is entered by name, the connection is always made over legacy IP. Mounting the same NFS share from the Linux shell works over IPv6 without problems.
Debug log: https://paste.coreelec.org/vGYqZN