Unless your code is strictly glue between those lower-level APIs, GC is still a benefit for the majority of it. And, on the other hand, the lack of it in FFI is, at worst, similar to C... except you still have all the other language features (like, say, null safety or pattern matching) at your disposal.

I'm genuinely curious as to what would make FFI in Rust easier than in C#, assuming an apples-to-apples comparison (i.e. use of "unsafe" and associated features in both cases"). Most complexity in P/Invoke shows up when you try to use it to automatically map data to managed data types; but in modern C#, you might as well just use raw pointers, stackalloc, spans etc.