So, I say, the portability advantage of C is a myth (not because it is not portable, but because it is none more portable than asm).

Well, in one project which was mostly assembly (since the compiler didn't know about half of the instructions of the CPU), I wrote the major part of a communications protocol in C, because it was mainly very generic processing (take a few bytes and twiddle some other bytes depending on the value of the first bytes, for the most part). Not only would that have been a major pain in the rear to write in assembly, especially if it was supposed to be efficient (the processor in question was pipelined and had some funny features like delayed instructions), it would also have been as non-portable as you can get.

The C code was pretty much reusable - no modification required, since all the machine-specific stuff was located in different modules.