Earlier quoted context omitted.
To develop a comprehensive test suite can sometimes be hard, especially for code that deals with say concurrency, multi threaded code , locks , 2d/3d physics , video , analog , hardware related , procedurally generated or ML (meta-language ) and the other ML (machine learning) etc. A lot of edge cases and race conditions would easily slip through, also a different set of edge cases or race conditions you never consid…
It might sometimes be hard but I have never seen a case where it was impossible (25+ years of experience dealing with undocumented legacy code more often than not).
Almost nothing is impossible to test yes, however to know and be able to mock the data for each test case can be extremely hard and at some point not worth the effort to even attempt.
Most I have seen these kind of systems doing is statistical testing with reference benchmark/ sample data, and maybe monitor real world feedback either telemetry or user complaints.