Problem:
The USNG field that is created with this tool has a length of 255 characters when it only needs 18. The reason this is a problem is that it doesn't match most other feature classes that have a 18-character USNG field already which makes it difficult to use any other tool that has field mapping options such as Spatial Join (Analysis), Append (Management), Merge (Management), Feature Class To Feature Class (Conversion), and Table To Table (Conversion).
Solution:
Reduce the length of the USNG field created by the Convert Coordinate Notation tool to 18 characters.
Note:
I realize this doesn't address a fully-qualified USNG value which normally has the datum at the end (i.e. (NAD83) ), but the tool returns a USNG parsed value with spaces between the GZD, 100k-m square ID, and the individual grid coordinates.