As a quick example, compare doing embedded work with a C static uint8_t[MAX_BUFFER_SIZE] alongside a FreeRTOS semaphore and counter for the number of bytes written, vs using Rust's heapless::Vec<u8, MAX_BUFFER_SIZE>, behind a embassy Mutex.
The first will be a real pain, as you now have 3 global variables, and the second will look pretty much like multi-threaded Rust running on a normal OS, but with some extra logic to handle the buffer growing too big.
You can probably squeeze more performance out of the C code, specially if you know your system in-depth, but (from experience) it's very easy to lose track of the program's state and end up shooting your foot.
So it's mostly about the absence of abstraction, in the C example? C++ would offer the same convenience (with std::mutex and std::array globals), but in C it's more of a hassle. Gotcha.
One more question because I'm curious - where would you anticipate C would be able to squeeze out more performance in above example?
For instance, how would better tooling help with storing a TCP buffer in global memory?