When comparing UiKit vs Cirrus, the Slant community recommends UiKit for most people. In the question“What is the best CSS framework?” UiKit is ranked 9th while Cirrus is ranked 23rd. The most important reason people chose UiKit is:
The code is pretty clean and follows well-defined conventions.
Ranked in these QuestionsQuestion Ranking
Pros
Pro Well architected
The code is pretty clean and follows well-defined conventions.
Pro Ready to use themes available
There are plenty of ready to use themes available from the official website. You can choose the theme that you want to use from the dropdown menu and then download the CSS, LESS or SASS file for that theme to use for the website.
Pro Easy to use.
When using UIKit classes, it is used with the ui- prefix which is very good. Components are explained straight-forward.
Pro Built-in animation capabilities
UiKit has some built-in animation features which can be used to animate various components.
Pro Great style even out of the box
UiKit has a pretty good and clean style even out of the box without any customization needed.
Pro Very customizable
UiKit's rather minimal style can be easily customizable to create an entirely new look to fit the needs of the designer.
Pro Extremely modular
Every aspect of the framework is designed to be modular, this way designers can easily choose which components to add to their stylesheet without risking to damage the overall style.

Pro Supports Flexbox and CSS Grid
Supports both Flexbox and CSS grid making it a great modern choice for designing web apps.

Pro CSS Only
No additional JS/jQuery required to use.

Pro Mobile Responsive
Extremely responsive and supports many smaller screens.

Pro Open Source
Open source and quite actively maintained on Github.

Pro Lightweight
Much smaller than Bootstrap with just as much flexibility.
Cons
Con Messy code classes
Nested classes become complicated to read to obtain desired result.
Con Not very popular
UiKit is not a very popular framework, especially compared to other options. As such it may be hard to find learning resources other than the official documentation or it may be more likely for development of UiKit to be dropped than for another more popular framework
Con Slow development
New features and updates trickle out over 6-12 month development cycles, bug fixes are more frequent but very slow and selective as well.
Con Pre-built starter templates are now behind a paywall
Easier to use Joomla! or Wordpress starter templates without paying money for it.
Con Closed development
Development is mostly done in-house and not publicly available.
