In Matrix client to server connections are just normal web-traffic, so that would require some deep packet inspection to block (pretty difficult but not impossible), however server federation uses a custom port and could probably blocked more easily.

XMPP by default uses custom ports and is quite easy to block by firewalls / routers, however it is possible to also use it via a normal web connection to circumvent that. That said, a lot of professional services use XMPP internally (emergency communication services, smart-meters etc.) and thus wide-spread blocking would be pretty disruptive for a lot of things. It is also possible to connect to and run a XMPP server entirely through TOR, which is pretty hard to block.

Neither is really great in that regard, but they are still harder to block then centralized commercial services (although Telegram tries to be "smart" by hosting their stuff on AWS etc, which also makes it hard to block as other stuff would be effected). Using XMPP through TOR should be relatively resilient though, but note that in some countries using TOR and other such services is illegal by itself AFAIK.