This is not really correct. The kinds of implementation dependencies usually encountered reflected processor architecture. The C standards committee and compiler community have created a situation in which different levels of "optimization" can change the logical behavior of the code! Truly a ridiculous state of affairs. The standards committee has some mysterious idea I suppose, but the compiler writers who want to do program transformation should work on mathematica or prolog, not C.
Compiler writers have to use program transformation to do well on benchmarks. Developers who don't prioritize benchmarks probably don't use C, and if they do they really shouldn't, because sacrificing correctness for speed is the only thing C is good for these days.
Speed isn't the only reason to use C. I often use C not because it's fast, but because C is by far the simplest mainstream programming language. All these UB warts notwithstanding, no language's interface comes as close as C's to a basic, no-frills register machine.
Yeah, we must have different definitions of simplicity, as I expected.
Just off the top of my head, Java has the following complexities that C lacks: exceptions, garbage collection, class inheritance, generics, and a very large standard library.
What definition of simplicity are you using when you say Java is simpler than C?
