Hi there,
New feature of click casting in key binding menu do not work with mouse forward and back buttons (side buttons). Is it a “feature” or a bug?
Additionally, I like the feature built in into UI, however lack of easy (without macros) customization just like Clique makes it not as useful as it easily could be. But it’s a start in a good direction.
Bug Reports, Feature Requests and Feedback Submissions are made from within the game, not via the forums, as the forum staff are not a part of the development team.
Adding features to the UI would indeed by a good topic for a suggestion in-game, but as far as the side-buttons go - those typically need to be mapped to a function/key within the controller software of the mouse first. WoW can then often recognize the mapped key, and thus enable the button for usage (e.g., my own mouse has its side-buttons mapped to numbers 9 and 0, since I can’t easily reach those anyway). 
They don’t need to be mapped to keys with external tools any more. You can use the new Click Casting UI to assign ‘Mouse 4’ and ‘Mouse 5’ (the usual back/forward thumb buttons).
Make sure you use the NEW click casting UI panel to do this. It works just fine for me. I have Riptide and Healing Wave mapped to the thumb buttons. There is also a ticked box option in the interface settings to make ALL spells work this way.