I don't think anyone has actual measurements on this, but at the same time anyone who's written and read a significant amount of Rust[1] can tell you that unsafe and RefCell aren't something you encounter every day (or even every year when it comes to RefCell).
[1] for my part I've been doing it full time for the past 7 years, and also teach Rust at university.
[1] for my part I've been doing it full time for the past 7 years, and also teach Rust at university.