Threema has a lot of vulnerabilities in the implementation of the NaCl cryptography library last I heard - basically the cryptographic algorithms it uses are good but they've been implemented in a sloppy way, leaving it open to side channel attacks (exploits to bypass the encryption without needing to break the encryption itself).

I have been meaning to look into SimpleX, seen it mentioned on Nostr a fair bit, I'll try getting around to looking into the details and testing it this weekend because I've been curious. But as of right now, can't give an opinion on something I haven't tried.