YUI Pros and Cons

Pros

• Clearer separation of client and server code.
  
• Works very well with traditional page-centric applications(unlike GWT). 
• Has decent graded browser support.
• SAP widget toolkit is built around YUI.
• Controls like Panels, Auto complete, Data table, tab view, slider, pop ups work very well and are easy to implement.
• Easy to adapt for skilled JavaScript developers.

Cons

• Lack of good IDEs leads to difficulties at development time for development & UT.
• Leaves you to manage how to translate your server-side return values to XML or JSON
• YUI Browser History does not support Opera. This is due to the fact that Opera does not update the `location.hash` property when using the back/forward buttons. It will be corrected in a future version of the Opera web browser.
• Developers need to use commercially available IDEs to develop applications in YUI. Eclipse plug-in is not free.
• Issue resolution for YUI library issues is difficult and forum updates not frequent enough

Cross channel support

It also offers two fundamental ideas, a) A broader and more reasonable definition of “support.” b) The notion of “grades” of support. The Graded browser provides an inclusive definition of support and a framework for taming the ever-expanding world of browsers and frontend technologies. 

But considering the way YUI architecture has been designed it is mostly up to the developer to take care of the cross browser issues in case of errors and incompatibilities. But the good point with YUI is as all code is in .js and it is open source, the developers can modify it to fix their problems (unlike GWT).