Seemingly, no one uses assembly nowadays other than to develop device drivers, or the very core of OS kernels etc. Anyone has knowledge of it being currently used for other thin
In C, at the fundamental level, it's pretty easy to see how a set of instructions becomes assembler (if you understand the underlying architecture). But sometimes, assembler can't be beat. Note that it is possible to write crappy, inefficient assembly code as easily as it is to write crappy C code. And assembler is way less maintainable. And it isn't portable anywhere.
And it all depends on the quality of the C compiler you're using. Some do better than others. Most compilers allow you to see the assembler code they generate. If you think you can do better, and the code segment is critical, do it. Otherwise, avoid it. Carefully crafted C can be very close to the metal, given a decent compiler.