alt Hacker NewsC isn't an iota more fundamental than say Modula-2. It's just more popular. And C is very bad at being a low level language. There are plenty of things that you can express in assembly that you just can't do in C. And then the platform ABI won't support it, since C can't support it, and it hampers other languages that don't have C's limitations.
A couple of examples what's missing from C from a low level perspective: Multiple return values (why can't I have a function using more registers than rax or the stack to push back more values?), sane varargs, stack manipulation (an enabler for coroutines and a lot more)
Generally, in an ideal low-level language I wouldn't expect to be hampered because the language is designed for a notional abstract machine that doesn't have things like an actual stack and contiguous RAM.