When I see people in other teams fighting with Maven, Jenkins builds for weeks somehow I cannot believe this.

I didn't say that Maven wasn't complex. I said there are fewer tools and less churn. The advantage of Java dev is that you can justify the intellectual investment in the tools because there are fewer of them and they are going to be around for a long time. The same cannot be said about client-side development at this time.

Grunt still exists, npm has been a common point for close to half a decade now... it's pretty solid... everything else builds from there. If you've gotten used to npm and cjs modules, that's been very consistent for a number of years now.

Would you use Grunt for new projects? Maven is still being actively used.

I thought Gradle was the new Maven. Before Maven it was Ant. There's even Buildr (though not as popular).