contrast this with emacs' syntax highlighting, that tends to break badly on any non-trivial use of backquotes. That actually shows the advantage of $() - it is

contrast this with emacs' syntax highlighting, that tends to break badly on any non-trivial use of backquotes. That actually shows the advantage of $() - it is simply easier to scan, both for highlighters and human readers. That said, there are many shells, that don't support $() and other random POSIX shell features. In fact, assuming that POSIX = portable is asking for trouble, because most systems actually aren't entirely conforming to POSIX (even when they are certified as being so). Another problem with relying on POSIX for portability is that in some places, the specification is worded in way that suggest one meaning and actually means something else (see for example behavior of glibc when operating on already terminated threads, which is POSIX compliant, but that this is correct is not immediately obvious from the specification).