When comparing Unity Sprite Packer vs GAF:Flash to Unity, the Slant community recommends GAF:Flash to Unity for most people. In the question“What are the best tools to create sprite atlases for Unity 2D?” GAF:Flash to Unity is ranked 3rd while Unity Sprite Packer is ranked 4th. The most important reason people chose GAF:Flash to Unity is:
GAF supports such Flash features as: Masks support; Filters support; No limitations on timeline; Conversion of shape tweens; Frame labels.
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Pros
Pro Create Custom Output Via Code
Set custom packing specified by code useful for those who have to follow a guideline on packed sprites.

Pro Comes built into Unity
The Pro license brings full 2D support within Unity without having to use an external program or plugin for creating atlases.
Pro Can convert 99% of what can be done in Flash
GAF supports such Flash features as:
Masks support;
Filters support;
No limitations on timeline;
Conversion of shape tweens;
Frame labels.
Pro No need to use large Sprite Sheets
With GAF converter you get a highly optimized texture atlas and a config file. You don't need to take trouble over cumbersome sprite sheets. You can easily modify an already working in Unity animation and play around with the animations parts whenever you need.
Pro Alows using Flash animations in Unity
You can convert Flash animations directly in Unity with the help of plug-in. The converted .gaf file consists of a texturte atlas and a binary config.
Pro Supports different frameworks
Full playback API supported in Unity3d, Cocos2d-x, Cocos2d, Starling.
Pro Light, fast, and optimised
GAF is lightweight and fast in a project.
Pro Uses dynamic text
GAF can use dynamic text in animation on some platforms.
Pro Can customize sprite animation
GAF can swap a sprite with another.
Cons
Con Doesn't work anymore
As of Unity 2017 it's completely broken.
Con Sound support available only in Premium version
GAF Converter supports sound but only in the Premium version.

Con Doesn't support sounds
GAF does not support importing audio along with Flash animations.
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