I'm not trying to compare xmpp (which I know a bit) and matrix (which I don't know about, really), but losing messages with xmpp is probably a thing of the past

I'm not trying to compare xmpp (which I know a bit) and matrix (which I don't know about, really), but losing messages with xmpp is probably a thing of the past. There were a good number of issues (the protocol originally didn't expect mobile devices that change addresses/connectivity all the time), but with a somewhat recent set of client and server you should be good.