Other people provided some info:

https://news.ycombinator.com/item?id=9772968

https://news.ycombinator.com/item?id=31133082 (article and discussion)

But TL;DR:

* It's hard to even parse, XMPP uses an uninterrupted XML stream. * The contents are often baroque and complex * Standards are a mess, and stuff that should be in core isn't * Data loss is possible * Protocol wasn't made for mobile devices * Multiple devices are terribly supported * Data loss is possible

From my attempts long ago, and other testimonials, writing an XMPP client is a full time job of solving weird problems that shouldn't exist in something better designed.