Well, it works with TinyCC/tcc where you can get order 200 millisecond compiles. While tcc does do most of c99, I do not think Nim requires anything past c89. libc-wise, it doesn't use even all of standard C. Minimal stdio/string stuff. You can easily access any C libs, though. So, a given program may have more dependency, but that is up to you (or to your Nim package deps).

Full disclosure, not everything works - different backends tend to have slightly different bugs/coverage, but this also applies to c++ and js backends. But I use tcc as my default backend (in my nim.cfg file) every day and only run into problems like a time or two per year.