One of the big challenges with IPv6 remains that many of the knows-just-enough-about-networking people, like support staff, often never received any IPv6 training (or, for that matter, even enough IPv4 training that they don't need to Google things that come up in real life). Another is that the weird, awful, everyone-hostile corporate "solutions" often break IPv6 in stupid ways (like load balancers and logging tools being unable to cope with minor changes and requiring a full configuration rework).

Things have definitely gotten better over time, though. The massive 90s style corporate networks will probably never transition, but smaller and more modern companies don't have that issue.

Apple mandating that apps are IPv6 compatible and various government legislation forcing companies to make their shitty middleware IPv6-compatible has improved things quite a bit so far. As uptake keeps rising, the need for technologies like STUN and TURN will slowly start decreasing, and as a result more and more people will end up in "untested" situations where not having IPv6 and falling back to legacy paths starts becoming a problem.