The big growing pain for XMPP was behavior when you had multiple clients connected simultaneously. Extensions were added to the standard that unified behavior a

The big growing pain for XMPP was behavior when you had multiple clients connected simultaneously. Extensions were added to the standard that unified behavior around which clients received messages (today we'd assume all of them but that was more up in the air at the time) and tracking of read status across clients in order to highlight new messages. But during perhaps the heyday of XMPP the extension was not universally supported by even some popular clients, so multi-client functionality could be confusing and inconsistent, e.g. messages delivered to only one client for no clear reason. This is a pretty well solved problem today but, well, no one is using XMPP.