It's simpler to get off the ground than webpack, less configurable (most stuff is done through babelrc), supposedly faster.
The big difference is using a HTML file as the entry point instead of a JS file, which means less weird stuff to do with CSS/pre-processors.
If you have a working webpack setup, don't worry about it.
If you're starting up a project that has an officially supported CLI, don't worry about it.
If you want fancy tree-shaking and only have JS, rollup has you covered.
Webpack is the most flexible (it's designed to be adaptable to all the weird stuff you might want to do).
Parcel is simple and designed for apps.
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