> There is nothing wrong with using this calling convention, except for those specific functions that need to have a variable number of arguments

I think it is a big pity that contemporary mainstream x86[-64] calling conventions (both Windows and the SysV ABI used by Linux-and almost everybody else) don’t pass the argument count in a register for varargs functions. This means there is no generic way for a varargs function to know how many arguments it was called with - some functions use a sentinel value (often NULL), for some one of the arguments contains an embedded DSL you need to parse (e.g. printf and friends). Using obtuse preprocessor magic you can make a macro with the same name as your function which automatically passes its argument count as a parameter-but that is rarely actually done.

OpenVMS calling convention-including the modified version of SysV ABI which the OpenVMS x86-64 port uses-passes the argument count of varargs function calls in a register (eax), which is then available using the va_count macro. I don’t know why Windows/Linux/etc didn’t copy this idea, I wish they had - but it is too late now.