JXA, like Scripting Bridge before it, was shipped crippled and buggy and abandoned once it was out the door. Apple finally disbanded the Mac Automation team and fired the PM responsible a couple years ago.

In fact, there are several production-tested Apple event bridges that totally wipe the floor with Apple’s failed attempts, but caveat emptor as I don’t do support:

http://appscript.sourceforge.net https://www.npmjs.com/package/nodeautomation https://hhas.bitbucket.io/welcome.html

We’ll see what happens when Shortcuts lands in 10.16, but I’d say the chances of Apple event automation having a long and healthy life ahead are 50/50, at best.