I guess it's not so much the language that is complex. Indeed, it's pretty simple for the most part (although there are archaic warts few people ever need and modern features that sometimes feel like C versions of C++ features). But the result has some emergent complexity because of the simplicity and lack of guarantees. When every line coupled with an antagonistic (i.e. optimizing) compiler has the potential to bite you and you no longer think about the apparent semantics of your code, but rather the semantics guaranteed by the C abstract machine that no one actually really knows, I guess the result is not very simple anymore.

I wouldn’t say it is complex, but since it is relatively low-level it doesn’t insulate from a lot of things that other higher level languages handle for you. Like the fact that an int has different sizes depending on the compile target. Or that one must remember to call this one function before calling this other function, otherwise Bad Things Happen (but it might only be visible way later, in production).