It's been well over 10 years since I worked with, almost 15. I remember issues with keeping multiple devices in sync, syncing them back up when a user comes back online, especially with multiuser chats. I understand that is probably better now, with carbons and archiving XEPs.

In general, it felt like, XMPP has too many "optional" features. The core protocol is tiny, but everything you need and want to make it useful is optional.