Isn't that true for any system that's been around for a few decades? Try implementing XMPP; which XEPs do you pick? It's a long list.[1] Try implementing email: there's probably more RFCs to exclude than include at this point, and what do you need and what is optional?
This is in fact one of the big issues with xmpp. Everything is sorta-kinda compatible but not really. And email is so getting so complicated that many people are scared of running their own server let alone programming one.
[1]: https://xmpp.org/extensions/