Which runtime checks are you concerned about, exactly?

Just array indexing? It would be interesting to see your criticism fleshed out.

I am not sure your criticism will amount to anything if array access is it, though. Posrgres is full of custom data structures and macros as it is. StringInfo and stuff like that -- they are not just "using the language", writing nice simple C code. Probably because databases really push you to use advanced techniques in places.