Every few months I turn on IPv6 at my house. I try to use it. I find random sites just not working, random delays accessing sites, and so on. Then I switch back to IPv4 and everything works.
I used to be a network admin, so I know how to configure networks. IPv6 zealots accuse me of incorrect config, doing it wrong, etc. Maybe that is the case, but if I, a sophisticated user, can't get it working well, what chance does a non-technical person have?
My assumption is they just deal with the issues and chalk it up to "technology sucks". But I know better. I've experienced the internet when it works, and I know when it isn't working right.
I think IPv6 is better in theory, and I look forward to the day that it is in practice. But today is not that day.
I suspect you forgot to implement a workaround for servers with broken pMTUd. The quickest test for that is probably to run `ip link set mtu 1280 dev eth0` on a client machine and see if it helps.
You'll encounter the same problem on v4, where it's just as difficult to fix as it is on v6. Why single out the latter?