scroll-hint | JS library to suggest that the elements | Script Programming library

I've got a gulp task that loops through a folder looking for sub folders and outputs a JavaScript file based upon the contents of each folder. Below is a more visual example.

• src
  • assets
    • scripts
      • critical
        • loadCSS.init.js
      • legacy
        • flexibility.init.js
        • picturefill.init.js
      • modern
        • connectivity.js
        • layzr.init.js
        • menu_list.js
        • scroll-hint.init.js
        • slideout.init.js
        • swiper.init.js
      • service-worker
        • service-worker.js

becomes:

• dev
  • assets
    • scripts
      • critical.js
      • legacy.js
      • modern.js
      • service-worker.js

This is achieved by reading the contents of the src/assets/scripts directory, then running a loop against each folder (critical, legacy, modern, service-worker) and sending the contents of each folder to a Gulp tasks which get merged together with merge-stream.

All this works great, except that once the tasks are merged back together, I want to trigger a notification if the compilation succeeded. If I try to pipe anything to the merged streams, it doesn't work. It just returns the merged streams, and never continues on.

If I un-promisify my PROCESS_SCRIPTS function and don't use merge-stream (i.e. only processing one manually specified folder), it works fine, so I'm at a loss as to what's going on.

Here's my full task: