It's not always possible to make the same security guarantees for these implementations. Software implementations of AES are frequently vulnerable to cache timing attacks, for example (e.g. [1]); even simple operations integer multiplication may not be constant-time on some architectures.
At some point this gets pedantic. Just not supporting anything but x64 and ARM64 means any project using ring or a dependency that uses ring can’t build elsewhere which turns ring into a land mine dependency.
> ... ARM, Amazon Web Services, Google, and Microsoft [...] should support the Rust community by letting their experts help the Rust community create FIPS-validated cryptography libraries written entirely in safe Rust that expose safe and idiomatic Rust APIs.
Rust's large corporate sponsors need to step up to make these crates more broadly suitable for production.
Alone how much Azure Sphere gets sold as high security device, and the SDK is C only. There is a new preview support for Rust, yet nothing announced for the stable product, only C on that one.