The major advantage of Matrix (not sure if DeltaChat can do the same) is the support for a lot of bridges, and how easily can you host it.

Matrix has a really good and robust ansible project, with which you can set up your own sever in like an hour, assuming you have a place where to host it (I use Hetzner for like 7$ a month) and a domain. Adding bridges and configuring the ansible only needed like changing 5 config lines at most, and it’s very well documented. It’s also super easy to maintain, I “just update” every few weeks and it’s so robustly written, that it lets me know what changed and what config I need to update. I never had an issue with it in the past two or three years I’ve been using it.

And then the bridges - I did not need to convince others to switch, becuase I run Discord, WhatsApp, Telegram, Signal and Messenger bridges on my Matrix server, which does bridge all of the apps into my Matrix server. Sure, they still get your conversations data, but at least you don’t have to have their spyware installed on your phone/PC and have it all consolidated into one Matrix app. I can also slowly convince people to switch to the more secure messengers like Signal, but don’t have to drop contact if they decide not to.