According to a quick look through the documentation, C code is called though a simple interface: (call-c-func "c-function-name" args …). OTOH it didn't appear that it's possible to call Scheme from C, which may be a disadvantage in some cases.

You are right, no mention of C++. It's not clear how much of a problem that would be. IIRC wrapping C++ in "extern C" more or less enables C++ functions to be called from C. Don't know about compatibility of Arduino libraries, but possibly this is how Microscheme FFI works.