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.
The main reason we chose Sencha is it’s full stack solution. We have an integrated backend that solves a lot of the infrastructure issues and Sencha was the only tool in the market that has only one library full of components. It is not fashionable to use it, as developers always love new technologies, but Sencha is the best for business applications and has a lot of useful tools.
Initial learning was hard, it has very specific tools and requires a basic knowledge that takes time to learn. But after that it's really fast and easy to do apps.
When it changed to version 2, servers had to be be upgraded due to the new requirements. Also it required to learn many things that were not backwards compatible.
It saves time by help us to create great applications in less time but very powerfuls.