When comparing CSS-On-Diet vs PostCSS, the Slant community recommends PostCSS for most people. In the question“What are the best CSS preprocessors/postprocessors?” PostCSS is ranked 4th while CSS-On-Diet is ranked 15th. The most important reason people chose PostCSS is:
PostCSS is 3-30 times faster than Sass (including libsass), Less, and Stylus
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Pros
Pro Fast to read and write CSS
Works like Emmet, shorting CSS keywords, but it's not limited only to writing. Also modifying and reading COD(CSS-On-Diet) files is faster.
Pro Easy to learn and use
Doesn't require programming skill to work with variables, mixins, media breakpoints
Pro Fast
PostCSS is 3-30 times faster than Sass (including libsass), Less, and Stylus
Pro Flexible
PostCSS allows you to opt-in to the features you need with plugins. This allows you to set it up to behave exactly like Sass, with nesting, mixing, extends, and more. On the other hand, it allows you to use plugins by themselves for things like auto-prefixing, minification, and more. You can even set up your own custom "stack" of plugins to do exactly what you like.
Pro Doesn't force designers to learn a new syntax
Rather than learn a different syntax, PostCSS allows you to write in pure CSS.
Pro JavaScript-based out of the box
Since it's basically CSS extended through JavaScript it works in the browser directly without the need to compile it beforehand.
Cons
Con It's difficult adjusting to different keywords
The keywords are shortened to 3 letters. For example, "background-color" becomes "bac" and "max-width" becomes "maw". These keywords are far less intuitive than their original form and make the CSS much less readable for those who don't know CSS-On-Diet.
Con Extremely limited adoption
CSS-On-Diet has just 7 stars on Github and a very small adoption rate. For an open source project this usually means less bugs reported, lesser documentation and few third-party learning resources.
Con Harder to install and keep working
The immense flexibility of PostCSS plus its current rapid evolution makes it harder to install, configure and keep running than the more monolithic and mature preprocessors.
Con Outdatet, plugins are often based on different postcss versions and don't work together properly
Con Some plugins need to run in a certain order
Some plugins can only work if initialized after some other plugins. For example, transforming and applying CSS variables needs to run before running a plugin which uses these variables inside conditional transformations.