My issue with XMPP was the lack of synchronization between clients. If my mobile was offline while I was chatting on desktop, mobile client would never have the same chat history as desktop and my conversation history would be fragmented. When I asked about it on some developer forum, they basically just said that was out of scope for XMPP and should be implemented in client. Which sort of makes sense if the client is a web client hosted somewhere but I wanted a thin desktop client like pidgin.im