Lack of detailed audits…only in this case specifically…does not imply lack of security and/or privacy.
The protocol that Signal uses, which is in fact firmly audited with no major problematic findings, plus the fact the client is OSS is generally enough to lower any concerns.
The server side software in production for Signal.org is not OSS. It will not be. You are required to trust the server to use Signal; because the protocol and the client renders it factually impossible for the server to spy on your messages. The server cannot read messages; or even connect who is messaging who if the correct client settings are used. (Sealed Sender).
Non-OS stats software in general is not automatically lacking in privacy or security, particularly not in this case where the affected software does interact only with software that is verifiably open-source and trustworthy in general due to the protocols and how they are implemented correctly in a verifiable manner.
S/MIME is insecure, outdated, depreciated, and should be discontinued; yet people don’t want to adapt or grow or change.
Because some organizations do use S/MIME; all email software is required to implement it, that is if they want to be adopted and used by said influential organizations.
OpenPGP and PGP in general is secure but suffers from usability issues and is often wrongly painted as user-unfriendly. (it’s really no worse than S/MIME, installing and managing keys is exactly the same hassle as it is with S/MIME.) The main issue is that some people are too lazy or resistant to change to adapt to it.