cxx is a huge improvement over manually writing bindings in C (with the inherent limitations imposed by C’s lack of expressiveness). At the same time, you still have to write a cxx::bridge to specify every boundary between Rust and C++ code, and the clients of on either side need to code against some generated code, not a simple .rs or .h file. It’s a huge improvement, but it’s still a lot more developer effort and boilerplate to use a C++ class from a Rust file than it would be to use that same C++ class from another C++ file.

cxx also can only handle a subset of the interfaces expressive in C++. If you haven’t written the interface with the specific goal of making it usable via cxx, it’s pretty likely you’ll have issues wrapping it with cxx.