Exactly, understanding correctness of the code. But also understanding of a codebase what its purpose is and what it should be doing. Add to that how the codebase is layed out. By adding more cruft the details fade away in the background making it harder to understand the Crux of the application.
Measuring performance is relatively easy regardless of whether the code was generated by AI or not.
Higher level languages did not hinder evaluating correctness.
Formal languages exist because natural languages are ambiguous inevitably.