Intuitive and Documented It is strongly influenced by jquery with many similar methods so you'll feel at ease developing with Umbrella. However there are also some improvements such as with .ajax(), .append(), etc.
Umbrella JS vs YUI opinions
pros, cons and recent comments
YUI is much more than a DOM manipulation tool, it features complex widgets and tools for dialog panels, data tables, drag and drop elements, etc.
Even the prolific jQuery library is a poor comparison without jQueryUI and a host of other plugins.