I wouldn't say they "support" C17. Perhaps with a big asterisk. Even with C11, they implemented _parts_ of the standard, but didn't ship some of its libs (threads come to mind). Same deal in C17. Any hopes of porting over standard compliant C code to MSVC are met with lots of additional porting work.

Also, if we do move the discussion outside GCC and Clang, then I don't know what to say man. Why not use GCC or Clang? Are there many UNIX out there not having either? Seems unlikely.