So I tried using this. It was nice while what I was doing fit inside of everything that was default settings and/or what maintainers used. As soon as I stepped outside of that I ran into things like having to hope someone implemented a Parcel plugin for some random transform (an issue that applies to Webpack, too, but Webpack has so many more users that you're guaranteed someone actually did make one that actually works correctly), or: https://github.com/parcel-bundler/parcel/issues/645. Having an issue on something as popular as Bootstrap 4 be open for 6+ months is pretty frustrating. I spent several hours flailing at this issue to no avail.
I switched to Webpack 4 and had all of my existing build system and some additional niceties working in <45 minutes.