It's great to develop business applications. Previously, we tried different technologies but we find it the most suitable for us. We also deploy a generic backend so we don't need to install anything new to implement new applications. This helps us to develop very fast and with very low effort.
Yii is very well suited if you love to program with Object Oriented PHP. This framework uses OOP very well and if you know this pattern you'll love it. The same applies for its MVC architecture and if you come from formal software development education. Also if you are in a bussiness enviroment and need a stable framework. This is the tool for you. It uses very formal scheme but I would like more open and hackable framework, and for this Yii2 is not a good option. Also, if you like to have bleeding edge technologies I don't recommend this framework.
I think is really easy to use, it's not perfect but many developers with great experience will know how to exploit this framework to create great apps!
It's has a really good support based on online documentation and official forums. But sometimes I wish there were a paid service where to report some urgent issues.
Each of the above platforms offer a specific solution. Sencha provides a complete solution that is a library of components as well as a framework to modularize your application so that it's better manageable. Once you get over the learning curve of the whole technology it's a breeze to implement new functionality within the application. We have an aggressive client who comes to us every other day with some new requirement, and sencha has been able to answer all of those without issues.