How may different compatibility deployments are there for IPv6?
6in4? 6to4? 6RD? NAT64, 6over4? Teredo?
Think any of those are fake names? Try again!
And that's just ONE piece of IPv6. Practically everything in the "spec" has at least 2 variants minimum, and its just a royal clusterfuck. When it is described as "protocol by comity", this is exactly the result, and its been a total pain in the ass to have anything reliable at scale.
You may be on one of the lucky ISPs that has a sane deployment and want to reply with "Well, it works for me!" - that's awesome, and I wholeheartedly mean it. That IS really awesome! But for the rest of us dealing w/ multiple ISPs in multiple regions, its a fucking shitshow to get anything reliable going consistently.
One IPS I deal with about 18 months ago entirely dropped IPv6 "support" - and now we can pull a single /128 address with no routing table at all. So we have an address that is entirely fucking useless, instead of having a normal block allocation which it was previously. Another ISP I deal with still uses PPPoE, and then uses 6RD over that, so the MTU is trash because both reduce the MTU size.
IPv6 is a fucking mess, and it pisses me off every day!