Most of the issues with Grunt and the like can be done away with by following good coding practices. i.e. keep your code outside the Grunt context and write unit tests for it. See the Thin Grunt Manifesto:

https://www.exratione.com/2015/08/thin-grunt-manifesto/

I'm not sure I buy fleeing to the command line as a viable alternative. You are in essence going to have to write a bash script at some point because the command will bloat beyond what can be sanely contained in a JSON property. Then you have to write tests for it. So why not just do that in Javascript, since you already have the infrastructure sitting there.

I think it is the case that most groups simply write lazy Grunt code in ways that make it hard to test. This is a chronic problem throughout the devops space; the code is frequently terrible. So don't do that. Don't drag the Grunt instance out into your code. Separate your concerns. Write testable functions and classes.