[speleding]

Not sure whether you would call this technical, but the difficulty lies in allowing third party access and still prevent spam.

The reason Whatsapp won out over competing services in the first place (over here at least) was that they managed to be both free and relatively spam free. All free alternatives quickly got subsumed by spam (even non-free SMS has a spam problem nowadays).

[ForHackernews]

Email has solved that problem already.

[showsover]

Claiming email has solved spam is a WILD take as 45% of current email traffic is spam.

[ForHackernews]

How much of that shows up in your inbox? I don't care about packets that are dropped by my firewall.

[lmz]

I guess if you count "silently blackholed by the other server with no recourse" an acceptable result then Apple / Meta can offer you that kind of interop too.