No identifiers, so your social graph stays private.
Also has an independent security audit.
To deliver messages, instead of user IDs used by all other platforms, SimpleX uses temporary anonymous pairwise identifiers of message queues, separate for each of your connections — there are no long term identifiers.
So then how do you pair with a connection? Do you send them (out of band like through email or something) some kind of key that they then accept? That sounds super annoying.
For me, the main distinction is selfhostability (although I have concerns about the majority keeping using the default servers, I could still ask a friend or family member to use mine).
Also I am a bit concerned about them pushing the "No identifiers" thing which seems misleading, since you're still identified, just per-conversation.