XMPP working on multiple devices is a problem long solved. Not losing messaging is a solved problem too. I can schedule you a demo if you wish.
A demo is worthless if experience doesn't prove it in the long run. And I have the latter, and it doesn't prove. Yes, I know it's probably me, or more likely my friends doing something wrong. One of them restricted Conversations from running in the background for example, non-techies do all sorts of silly stuff. But then again, it shouldn't break with non-techies either. Nothing breaks with them aside from XMPP. Not even WhatsApp which is based on XMPP afaik