About using C code:

• there obviously the potential bugs and vulnerabilities in the C code itself and in the code wrapping the C code.
• specific C toolchains, which makes things hard when you do cross compilation.
• you (sometimes) need to deploy the dynamically-linked C library (OpenSSL). It prevents you from using secure FROM scratch container images.
• can't compile it for WebAssembly.
• maintenance! it's hard to review a 2000 line implementation of an encryption algorithm

Pure Rust cryptography is usually around 10-25% slower than an ultra optimized C or assembly code.