UX issue, and UX issues are often downplayed by engineers, leading to adoption failures.
Another such example is SELinux, which would have prevented so many vulnerabilities from being exploited, but whose poor UX also caused everyone to disable it at install time.
SELinux's UX was significantly improved many years later, but already too late to change ingrained opinions. There are a lot of ingrained opinions about IPv6 too.
Conversely it means people who have ISPs that do IPv6 just have IPv6 and don't need to turn it off. Because it just works. The other day my IPv4 was down and I didn't even notice.
I don't expect any ISP to do IPv6 today and deploy routers with a flow label bug... Those types of bugs no longer go unnoticed.
IPv6-only ISPs might hit other issues, though. They have to bridge to IPv4 somewhere.
> SELinux's UX was significantly improved many years later
in what way?
Most of what people see as "SELinux" is actually the default policy, which started out as way too strict. Then SELinux-enabled distros such as Red Hat moved to a policy that only applies to system services, and leaves user-launched binaries as if SELinux was disabled.
And even for system services, you can disable SELinux for one service (permissive mode) and leave it enabled for the rest.
This has been the case for more than 10 years, but the damage was done. It's now very hard for users even considering learning the basics (which are not hard).