From my experience the biggest pain point gets the least amount of discussion in this post and that is the client landscape. I'm on iOS so I tried Monal and Siskin IM as my primary mobile clients and Movim/Dino as my desktop clients.
While I truly appreciate the effort the maintainers put into these tools I have to admit that the UX for Moanl and Siskin IM leave a lot to be desired and if you're used to something like Whatsapp or Signal it will prove a significant barrier to convincing friends and family to adopt your new platform.
I frequently encountered UI bugs and missing features using the iOS clients and for the life of me could never get notifications working reliably on mobile (a constant bug bear for users in the Siskin and Snikket user base).
For calls and texts on mobile, I don't think I ever managed to get a reliable notification unless the application was open on the screen on iOS. This meant that I frequently missed important calls or texts when out and about. On the desktop, Dino did manage to at least always alert me when a call or a text came through, but answering the calls on was always hit or miss on my laptop for reasons that allude me.
Interestingly the most full featured and reliable client turned out to be movim, which has the caveat of being a full featured formum/social network with an xmpp client embedded. On movim I would reliably get notifications for calls and texts and could "answer" calls in any browser connected to a microphone easily.
Another thing to note if you're looking to replace your phone service is that 3G/LTE radio is very tolerant to maintaining a reliable connection on the move, jumping between cell towers. The same cannot be said for an XMPP based call, and you will encounter significant latency and drop outs trying to hold a conversation driving or on transit.
I eventually gave up on the venture after missing a few too many important notifications, but if I was to go defiantly go all in again I think I would focus on self hosting a movim instance as my base "client", as it was the most reliable and easy to use of all the ones I tried. For mobile, I did hear good things about Conversations on Android but never got around to trying that one.