When comparing SmoothMoves vs Unity 2D, the Slant community recommends Unity 2D for most people. In the question“What are the best 2D skeletal animation tools?” Unity 2D is ranked 6th while SmoothMoves is ranked 12th. The most important reason people chose Unity 2D is:
2D game creation was a major feature request from the Unity community and was added with version 4.3. 2D is provided in both the Pro and Free distribution of Unity.
Specs
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Pros

Pro PlayMaker integration
Developers echo17 provide Playmaker action scripts for SmoothMoves.

Pro Texture swapping and tinting at runtime
Animations can be re-skinned in several ways, entirely on the fly.
Pro Built into Unity 4.3+
2D game creation was a major feature request from the Unity community and was added with version 4.3. 2D is provided in both the Pro and Free distribution of Unity.
Pro Integrates with Mecanim
Mecanim brings state machines and blending to 2D bone animation. The state machine editor allows for designers and programmers to visually create a Finite State Machine (FSM) to control when animations should play. Mecanim also allows for blending so an animation can transition smoothly between two states without the need of in between frames made by an animator.
Pro Sprite Sheet and Bone based animation supported
Both types of 2D animation for game development are supported in Unity’s system and can be used interchangeably in the timeline.
Pro Easily change sprites dynamically
Usually used for character customization, programmers can change any sprite in a bone animation at runtime easily by referencing the bone and loading the new sprite from the resources folder.
Cons

Con Bone hierarchy is not collapsible
When working with a complex skeleton, the entire tree is always visible. There is no way to hide bones under their parents.

Con Unity Middleware
SmoothMoves is not a stand-alone application. It runs entirely within the Unity Editor, and is not compatible with other engines.
Con Lacks critical features
- Vertices can't be animated, so you can't have ANY organic feel (like... lungs breathing).
- Parent bone can't be animated without affecting the children. This is especially impeding for organic feel, again.
- No option to show & unshow assets (or it is hidden), like for switching weapon on your character for exemple.

Con Poor script interface for texture atlases
Accessing individual sprites within an atlas texture is possible at runtime, but requires use of the Resources folder subsystem.
