We should distinguish code that's correct using the borrowing metaphor but won't pass borrowck in current Rust (such code will inevitably exist thanks to Rice's Theorem) from code that's not correct under this metaphor but would actually work, under some other model for reference or pointer types.
Because Rust is intended for systems programming it is comfortable (in unsafe) expressing ideas which cannot be modelled at all. Miri has no idea how the MMIO registers for the GPIO controller work, so, too bad, Rust can't help you achieve assurance that your GPIO twiddling code is correct in your 1200 byte firmware. But, it can help you when you're talking about things it does have a model for, such as its own data structures, which obey Rust's rules (in this case the strict provenance rule) not "memory" that's actually a physical device register.
Because Rust is intended for systems programming it is comfortable (in unsafe) expressing ideas which cannot be modelled at all. Miri has no idea how the MMIO registers for the GPIO controller work, so, too bad, Rust can't help you achieve assurance that your GPIO twiddling code is correct in your 1200 byte firmware. But, it can help you when you're talking about things it does have a model for, such as its own data structures, which obey Rust's rules (in this case the strict provenance rule) not "memory" that's actually a physical device register.