I would be scared if they weren't using C (or a derivative like C++).

C is a highly deterministic language. It has no "features" per se. If you point to a line of C code, I can tell you with very high certainty what assembly opcode(s) it will compile to. The only thing C does that might be considered a "feature" is memory abstraction, which is kind of important for any large programming project.

Now, if you're trying to advocate using Java or Python or something, you've obviously never worked on embedded software. Not only does any very-high-level language introduce much higher resource requirements, but they are also much less deterministic (which is BAD for spacecraft).

Proper C code is not less stable than any very-high-level language. In fact, there is a lot less that could go wrong.