Comparing the resilience of instant messengers Briar, Session, XMPP, and Matrix.
In general, all statements made here are fact and can be confirmed in the projects' documentation. Information found elsewhere is cited.

Server Outages and Local Internet Outages:
Briar, Session, XMPP, and Matrix are all decentralized networks. That means that there is no central server that can go down that would cause the entire network to fail. So that's already amazing.
In a federated model such as XMPP and Matrix, the resilience of the network is dictated by how many servers there are and how distributed the users are among them. More distribution/servers = more resilience. XMPP is more resilient to server outages than Matrix simply because there are more servers. What Briar and Session have in common is that they use a network of random, alternating servers to pass communications to a contact. Meaning that if one or even many of those servers go down, there are plenty left to pass your data across.
In summation, XMPP and Matrix are resilient to some servers in the network going down, but won't work if your server has gone down, while Session and Briar would still get the messages across. If you're confident running your own server or just like the idea of being the one responsible for any outages, self hosting XMPP or Matrix might be the route for you.

Country Wide Internet Outages:
They are some messengers that are inadvertently advertising to work during this kind of outage. This doesn't make sense. The countries that are getting their internet cut off
wouldn't benefit from these messengers because there's no way to even send or receive a message there. It doesn't matter if a messenger is decentralized if you can't reach a server. And the rest of the world wouldn't be affected by these outages anyways because most, if not all centralized services and decentralized "nodes" are not based in countries with frequent internet outages. (1)
But there's an exception: Briar can work without the internet, using shared WiFi routers and Bluetooth.

Censorship:
Briar, Session, XMPP, and Matrix are all fairly resistant to censorship. The internet traffic from these services can be identified by a government/ISP, and shutdown. It is more challenging than with a messenger like Telegram or Messenger or even Signal that have more well known IP's and large user bases, but still possible. I would say that a self-hosted at-home XMPP/Matrix server is best to protect against this as Briar uses well-known TOR IP addresses and Session's seem to be getting more well known. Feel free to give me your thoughts though.

Takedown Orders/Targeting Individuals:
Because Briar and Session generate "accounts" locally, on the device, they cannot be closed on the server side and it would be financially prohibitive to restrict an individual's access to the network. These accounts can be created rather quickly, sometimes being referred to as ephemeral accounts.

DNS Outages:
Briar doesn't rely on the DNS system, it uses TOR when using the internet to pass messages, making it resilient to DNS outages.

If there's anything I left out that you feel to mention, let me know.

(1) https://www.accessnow.org/internet-shutdowns-2021/

-Jami is not mentioned as it is not working reliably, at least on Android.
-CWTCH is not mentioned as I have not researched it enough. I like Briar's code design more, as a plugin can be written to pass messages on another transport. Haven't seen much documentation/info for this service
-Email is not mentioned as it is too centralized and restrictive (around self hosting/getting messages through) to be considered as a good federated option.
-IRC is not mentioned as it seems to be very centralized and I have not researched it enough.
-Tox is not mentioned as the user experience on mobile is not polished.