The Telerik API is lacking in a ton of areas. I just finished writing a very large enhancement for my employers' RadMenu. A lot of my time was spent looking through the RadMenu and the RadMenuItem's client side properties using DevTools, because many are not documented on the Telerik site.
Don't even get me started on the fact that you can't use JQuery to traverse Telerik's controls or Webforms in a whole for that matter.
There are some Pro's to the RadControl AJAX toolkit, but please improve your documentation. Expand Direction enumerators, Group Settings, Set Flow, _Element, ClientWidth and ClientHeight and 100s more. So, these Pro's are more evident to your user base, because presently it is a giant CON.