I've seen some pretty phenomenal flight software written in C. Well-architected, solidly implemented.
I've also seen some pretty horrible flight software written in C.
Naturally, and as with all software, clear requirements, avoidance of "feature creep", and a clear vision of the lead developers is what distinguishes the good from the bad. So these are critical components of "safe" software development, irrespective of whatever "safe" language you're using...
Edit: On second thought, strict adherence to a uniform coding standard is incredibly important too. Even though it's just cosmetic, it holds the developers to a higher standard, and strongly discourages breaking the rules to get things done quickly.
I've also seen some pretty horrible flight software written in C.
Naturally, and as with all software, clear requirements, avoidance of "feature creep", and a clear vision of the lead developers is what distinguishes the good from the bad. So these are critical components of "safe" software development, irrespective of whatever "safe" language you're using...
Edit: On second thought, strict adherence to a uniform coding standard is incredibly important too. Even though it's just cosmetic, it holds the developers to a higher standard, and strongly discourages breaking the rules to get things done quickly.