I've run that on all manner of devices and infrastructures and it has never told me I will have any problems on World IPv6 Day. That's not because it is inacurate, it's because the vast majority of setups will continue to work perfectly on June 8th.
You might experience problems in environments where someone has been playing with IPv6 and left it half-broken (or, in my case, tried to be too smart and used DNS server on Cisco router ;)