When comparing Unity Sprite Packer vs TexturePacker, the Slant community recommends TexturePacker for most people. In the question“What are the best tools to create sprite atlases for Unity 2D?” TexturePacker is ranked 1st while Unity Sprite Packer is ranked 4th. The most important reason people chose TexturePacker is:
Drag and drop sprites onto the canvas, change the settings, then output.
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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 Simple Interface
Drag and drop sprites onto the canvas, change the settings, then output.

Pro Multiple Atlas Output Formats for Unity
There are three different outputs for Unity to read the atlas; JSON, Texture2D, and 2D Toolkit.
Cons
Con Doesn't work anymore
As of Unity 2017 it's completely broken.
Con Pure functionality in the free version, you can't even set margins
The full price is too high for such a simple app.

Con No direct integration
TexturePacker is an external third party application with no Unity Inspector interface.
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