> If for any reason a C++ project integrates Rust in a way that is permanently through an FFI, then the development costs to use the unsafe non-ergonomic FFI are worth the effort for this unique situation.

I don't follow this train of thought; could you elaborate? Why would I want more unsafe code in my project because it's going to be there for longer? If anything I'd want the opposite -- longevity of the unsafe code being a reason against wanting more of it sitting around.

FWIW the primary use case of CXX in my work codebase is in long term hybrid-Rust-C++ projects in which dozens of libraries using CXX (in both directions) are going to be around for many years.