Shape keys do not work with the action editor?
In Blender, the Action Editor primarily works with armatures, leaving shape keys somewhat disconnected from the animation workflow, especially for Unity projects.
With this video I will guide you through the process of setting up these drivers so that you can use shape keys in your next animation project within the Unity game engine with you Blender models.
The solution
The solution involves utilizing Drivers to link shape keys to non-deformation bones within the armature. Because drivers can be added to bones we can use them to "drive" the shape keys, thus being able to animate them per "action" for unity.
Benefits
This method enables you to control shape key values through bones, making them compatible with the Action Editor and thus allowing seamless integration into your Unity animations.
By attaching shape keys to bones via Drivers, you establish a direct relationship that facilitates smooth animation transitions and cohesive actions within Unity.
Negatives
Honestly, there are few scenarios where this methodology won't work for you. You just need to be mindful that your model doesn't not have any other modifiers other than the Armature modifier, as shape keys are not able to be exported while the "Apply Modifier" option is checked in the blender export options.