They quickly become unwieldy on non-trivial APIs, with hundreds of definitions across dozens of files and with macros to boot. Naturally people would still get the job done but it's beyond simple.
I have used it successfully against header files for Win32 COM interfaces generated from IDL which include major parts of the infamous "windows.h". Almost every type is a macro.
Not types, functions. Where the macro is essentially a forward declaration but the implementation is deep inside the code and is not exposed via headers.
