Even Matrix isn't perfect. I would consider Signal and Matrix to be pretty secure and recommended for activist organization, until the US decides to force Signal to open a backdoor into its end-to-end encryption. Signal only provided the account number, last connection date, and account creation date (in unix time format, lol) when the California grand jury issued a subpoena. Signal has also threatened to leave the US and the UK if they passed their anti-encryption bills.
Signal is not without criticism, though, considering their controversial cryptocurrency project.
US used RISC-V BAN!
US became confused!
It hurt itself in its confusion!
It's super effective!