Rework is designed to extend the functionality of vanilla CSS.
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Pro Extend native properties
Through transforms you can modify existing properties to give them new attributes and options, so instead of managing messy mixins, you can add a simple new attribute where they make the most sense.
Pro Unrestricted potential
Because Rework plugins are done in code, there are no limits to what they can do, and they tend to provide more advanced functionality that would be harder to implement in other preprocessors, such as file I/O and custom logic.
Pro Built around plugins
Rework isn't a language for compiling to CSS but rather a library around parsing it and transforming it. For example, a vendor prefix plugin will inject prefixes around needed properties so you don't have to muddy up your CSS dealing with it.
Because Rework is built around plugins at its core, it makes for easier plugin writing if you find you want to add in new functionality.
Pro Can work with other preprocessors
Although you don't have to, since Rework works on vanilla CSS, you could use another preprocessor that has a syntax you enjoy more before applying Rework's transforms.
Pro Allows for customized properties
Rework plugins can recognize custom properties and transform them via plugins. This allows you to keep your CSS clean and expand its functionality in a native feeling way, without having to learn a bunch of new language constructs.
Con Replaced by PostCSS
Rework basically solves the same problem as the more popular PostCSS.
Con Difficult for beginners
Rework has a more involved setup that can make it an intimidating first option for beginners to css processing. As Rework is built around plugins, the documentation can't be found in one spot. The quality of documentation also varies between plugins.