Hummm, not my experience. The first time I ran AFL on one of my C library (parser/converter of EMF files, really easy to mess up), the result was quite frightening. IIRC, it took only 20 seconds for AFL to find 10 crashes.
Ever since, first I'm more careful, second I run AFL from time to time, and it happens regularly that it catches some edge cases I was not able to see.
I love programing in C, but I definitively know it's easy to make mistakes in it.