It may be missing references. If you open the assembly in the UG viewer are all of the pieces there?
The Unigraphics translator imports the Parasolid information of a Unigraphics® part or assembly into a SolidWorks part or assembly document. Only the Parasolid information is extracted, not the proprietary feature information of a Unigraphics part.
You can import Unigraphics compressed part files.
What version of UG is the file? Maybe it is a newer version then SW can open.