A big difference is that using sed is producing a single known output that is clearly evident, it's much less clear what all the loops and calls is producing, it may even be an example of premature abstraction.

I don't think relying on shell commands is a bad thing though, it's just part of the unix philosophy, every build tool in existence will call external programs like the compiler itself. I doubt you could find another build tool that can replace values as simply as sed. The shell command would actually be unnecessary on newer versions of make.

Totally agree on the evaluation model though.