In proprietary software it's different. Proprietary software is less vulnerable to infusions of backdoors from untrusted sources and side channels. Proprietary software can only be audited by the developers themselves, and it will depend on the kinds of resources the developers can bring to bear directly. Companies that can afford it can dedicate large teams to reviewing and testing their codebases.
That open source code can be audited by third parties is only relevant if it actually happens, and otherwise you have only a false sense of security.
That open source code can be audited by third parties is only relevant if it actually happens, and otherwise you have only a false sense of security.