I've been using Matrix for years, but now only as a replacement for IRC. The encryption key handling has always been cumbersome and flakey, and too easily broken by users. Not compromised "broken", but locked out "broken." It's been like this for years, and while the UI has improved, it's still too hard for casual users to confidently use; I've given up hope that it'll ever get to a point where I can recommend it to friends who don't give a fuck how it works, and who aren't interested in spending a half hour figuring out how to set things up - they just want it to work. So many encrypted messaging systems have done this correctly, I dispair that Matrix can't (it's a common issue with all clients, so I blame the design of the protocol).

Edit oh, I also wanted to say I'd also been disillusioned with Matrix when I realized I couldn't run my own server. That is, I technically could; I just couldn't afford to. Synapse is a hot mess of a server, but it also just pounds on the CPU and requires massive amounts of disk space (over time). Matrix is designed such that all content for channels joined by any user is replicated to the user's home server. It's a questionable design decison, at best, but a consequence is that regardless of the server software, the storage requirements make running a home server cost prohibative. Compared to, say, running an xmpp server, which could be done effectively on a Pi.