I'm having a little trouble imagining a situation where unit tests for a C codebase are appropriate that would be stymied by having to write the tests in C++. (Maybe relying on struct layout?)

Because C hasn't been a subset of C++ since C99, there are plenty of case where trying to do C in C++ is awkward or just not possible with standard C++:

First, you'd have to wrap your header inclusions in `extern "C"` to disable mangling, then you would have to make sure you static_cast all your pointers where normally `void*` conversions would have done its job.

Furthermore, all interfaces relying on designated initializers and compound literals are broken unless you decide to compile in nonstandard GNU C++.

And there are more incompatibilities, such as using `static` or `const` in array parameter declarations or using VLA in macros which are not recognized by C++.

It's all about using the right tool for the right job, ultimately.