Rework vs cssnext

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.

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.

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.

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.

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.

cssnext is a PostCSS plugin, which makes it pretty easy to use for people who are already using PostCSS.

Because the parser/compiler can function in a web browser, it can be used with systems that cannot run similar technology on the server. For example, you could build a WordPress plugin with a front-end application that transforms CSS.

Since css-next only adds new CSS features in a way that all browsers can support it, it's still CSS. So there's no need to learn any new syntax.