Undefined Behavior is the obvious one. It's everywhere, with more subtle rules than most people realize, and yet it's invisible. Taking advantage of actual port

Undefined Behavior is the obvious one. It's everywhere, with more subtle rules than most people realize, and yet it's invisible. Taking advantage of actual portability of C adds dealing with variable integer sizes, and requires even more care around implicit type conversions. Lack of any thread-safety in the language shifts that complexity onto programs and programmers. Similarly lack of memory safety in the language itself pushes the complexity to the tooling — analyzers, sanitizers, fuzzers, hardening. Writing C properly requires a lot more stuff that isn't so simple. Plus there's a lot of dumb pointless complexity of just dealing with needless compiler and platform differences, build systems, and dependencies. Half of the time leading to a paradox of C being super portable, but Unix programmers telling Windows users their OS and compiler sucks and isn't supported, and vice-versa. BTW "simple" in English is ambiguous and could be interpreted as either synonym for "easy", or "consisting of few/basic parts". C is the latter. Actually writing proper safe reliable C requires skill and experience way beyond the spartan basics of the language itself.