Someone made a compilation of academic reviews and blogposts here: https://community.signalusers.org/t/wiki-overview-of-third-party-security-audits/13243 but none of them seem to be real security audit reports, ex. compare with real security audits to Delta Chat: https://delta.chat/en/help#security-audits
Sure is. It’s only that in this case you are sure that your messages are sufficiently protected, so you can send them over a untrusted service.