PPI is nice for static parsing, but this should work in any dynamic context. (Just think about an imported sub using prototypes, and hence dynamically altering the parsing rules)

Let me pick one out of all the problems source filters have:

Combining them with others, or even nesting them.

They try to parse a whole file globally and introduce code at a distance. The next filter sees that output and is confused. The resulting problems are a nightmare to debug.

This can't happen here.

That's why macros are such a powerful tool in the lisp world.

Of course they should be used with care and have pitfalls ( see "hygienic" macros), but that's a problem all code generation has.

There are many longer elaborations about the limitations of source filters, I'd like to point you there.