The main downside to putting scripts into package.json (or NX's project.json) is that you have to wrap it in JSON. Which is fine for simple commands, but when you start adding stuff like quotes or multi-command commands it starts to get a bit busy.

I quite like make or just as a task runner, since the syntax / indentation / etc overhead is a lot lower. I haven't yet tried to introduce it in any JS based projects though, because it adds yet another tool.