Techlore gives an interesting short video on why he doesn't recommend SimpleX right now.
I think it's an interesting idea, but solves a different problem than the obvious competition in Signal. Signal is not without fault of course, but it does what it says on the tin, gets you secure messaging with your contacts. It doesn't hide that you're talking to your contacts and relies too much on the OS security it's running on IMO. (doesn't lock itself anymore or use a password to launch)
I haven't tried SimpleX yet, but my reading of it is 2 problems.
- like Techlore, it's too new. Let's get some experience and audits under our belt.
- it's worse than the fediverse for non techie people. It reads like manual key exchange, which while secure, is basically unusable for most people.
- the problem it solves isn't one most people have. Hiding your social network... The people you know and communicate with is only slightly desirable for the average person and near impossible to do in today's world. And if you're not taking on govt level threat models, it's irrelevant. For most people interested in privacy, something like Signal keeps their contents of conversation private, and also keeps the people they are communicating with private from advertising and ISPs.
- if you do want SimpleX hiding of who you talk to also, there are tools that have been around for quite a while that you could use, with the assumption you and your friends have the tech skills. The needed skills are maybe slightly more than SimpleX... Debatable.
Anyway I will keep watching SimpleX too, but I doubt it'll be something I can get the people I communicate with to switch too. It's been a lot of work to get them to use Signal, and that used to be a drop in replacement for SMS(still annoyed by that going away).
I't is made for different purposes: Matrix is built around communities and groups (like Discord) whereas SimpleX tries to secure direct messages and info channels (like telegram). It has no identifiers unlike Matrix so you can create a whole new "account" for each conversation. This makes it very easy to talk with strangers. Also they plan implementing a meshed network approach (like Briar) so that you dont have to rely on your own interent connection.