I have seen this idea floating around… personally I would rather write JS than learn another weird file format that I will have to Google every single time I mess with the Makefile, but the idea is decent.
I would say the documentation and google results for a decade old tool would be more available when you try to do something a bit complex.
Yes -- and then I proceed to use the thing every day. I wouldn't be adjusting a Makefile very often so I would keep forgetting whatever I "learned" every time I changed it.
If you are using Grunt everyday, doesn't that mean you are modifying it? Then you would be in the same exact place as with a Makefile.