Diodes are primarily an information leakage solution: they permit data flow in one direction only, and typically in the direction of increasing sensitivity. Although this prevents sensitive information from leaking into a less sensitive (and less protected) network, this poses two problems:
Trust: if we interpret “high” and “low” in terms of trust rather than sensitivity, we’re moving less trusted data into a more trusted system, which is a security risk.
Accessibility: we might want to export redacted or sanitised reports to a less sensitive system. The diode is data-agnostic: it can’t prevent sensitive data from escaping from the high system by masquerading as a sanitised report.