First of all, still I am a fan of YUI libraries. It is an open source client-side library and provides rich set of UI  components (like Datatable, Uploader..etc.,) and it was actively maintained by YUI team. but now  YUI announced and  decided to stop all new developments and will do bug fixes only if anything critical to YUI properties. So I will not choose/recommend YUI for coming  projects.

Below is a important points from YUI announcement.

The consequence of this evolution in web technologies is that large JavaScript libraries, such as YUI, have been receiving less attention from the community. Many developers today look at large JavaScript libraries as walled gardens they don’t want to be locked into. As a result, the number of YUI issues and pull requests we’ve received in the past couple of years has slowly reduced to a trickle. Most core YUI modules do not have active maintainers, relying instead on a slow stream of occasional patches from external contributors. Few reviewers still have the time to ensure that the patches submitted are reviewed quickly and thoroughly.

Therefore, we have made the difficult decision to immediately stop all new development on YUI in order to focus our efforts on this new technology landscape. This means that, going forward, new YUI releases will likely be few and far between, and will only contain targeted fixes that are absolutely critical to Yahoo properties.