in a world where iOS users won't install another free app from the app store because they already use iMessage, matrix is like asking for your friends to perform calculus just to talk to you.
Signal is very much in the same area of: "trust us".
With a caveat that they also say: "here's a bunch of information on why you should: but you can't really verify any of it and we have proven bad faith before- also we have an army of people who will pile-on if you call us out for not being actually verified, so, just trust us- we are the secure messenger and all those scary things are just so we are easy to use".
Definitely not true. Facebook literally censors private conversations. You simply can't send certain text strings to your friends. That is far more dangerous than relying on a third party that claims to be protecting your privacy. Especially since all signs point to them being honest.
To be clear, this is in private conversations. Not just posting publicly on Facebook or w/e.
To their point, there are benefits to federated systems. But I've yet to see a federated system have moderate to large usage without becoming centralized. Think email. And until this problem can be solved you're still left with a "trust us" problem. There's no trustless system out there, yet. But hopefully it comes in the future. In the meantime, signal is the best if you also want to communicate with anyone that can't tell you if a stack is FIFO or LIFO (or even know those acronyms).