What are potential disadvantages of using extern "C"?

Question

I have read

• When to use extern "C" in C++?
• In C++ source, what is the effect of extern "C"?
• Why do we need extern "C"{ #include <foo.h> } in C++?

However, one question that I have not found an answer to: are there (potentially, future) disadvantages to using extern "C" (e.g., on as many functions as possible)?

To be more specific: Is there any disadvantage in adding extern "C" to functions whose interface only use C functionality; in other words, those that do not use the features listed in @k-five's answer?

Answer 1

The disadvantage is that you can only use features in the interface to extern "C" functions that are also available to C functions.

That means:
1. you can't use default values for function arguments,
2. you can't use reference arguments,
3. you can't pass C++ classes by value (including smart pointers),
4. you can't pass enum class arguments,
5. you can't pass bool without converting it to int,
6. you can't overload such functions, and probably more that I can't recall at the moment.