The greatest weakness of the XMPP is... it's main advantage - it's extensibility. Because of that, which you can "pick and choose", it's more difficult to offer

The greatest weakness of the XMPP is... it's main advantage - it's extensibility. Because of that, which you can "pick and choose", it's more difficult to offer "consistent IM solution" because there is no one party/company that offer complete environment (server + clients for all platforms). XMPP evolves and is developed by volunteers (and developers contributing to XMPP software) and there has been a lot of work done to improve it (some say "catch up with IM landscape"): chat synchronisation (same conversation in all clients), E2E encryption (OMEMO), Audio/Video Calls (with Jingle), improvements on mobile (push) and groupchat (persistent membership / MIX). Surprisingly enough, XMPP is quite popular and there is a lot of deployments but... XMPP is not exposed or advertised as being used, so you may be using XMPP without even realising it. (sorry if this is a bit vague but I'm not sure I can give more details as I'm involved with one XMPP project)