Hand-written [...] parsers can trivially do [whatever] and ... give a useful error message ... Of course with arbitrarily complicated grammars that's not doable

Never say never...

If:

• Grammars are analytic, and such that grammars and parsers are the same thing; and
• The formalism makes it easy and productive for mere mortals to read, write, and maintain grammars/parsers of arbitrary complexity while intuitively avoiding turing's tarpits (space and time); and
• Provide expected conveniences for abstraction of facets such as error message construction, and zero overhead construction of subsequent compilation phases;

Then the classic disconnect is resolved and by definition grammars/parsers are guaranteed to be:

• In sync;
• Capable of being as good as can be specified by any turing complete language.

cf the section What are Raku Rules / grammars? of my StackOverflow answer to What's the real difference between a token and a rule?