Joe, I would start by looking at the list of contents. In the case of my own Open Content, at least, it is essentially a word for word transcription.
I can't comment on the artwork-- I don't own most of the original Mongoose material, and presumably that's the only artwork they would own the rights to reuse.
If you already own the source material, the primary benefit of the UGDC is portability.
I bought my copy mostly to see if they had given me the correct copyright attribution in the OGL-- they didn't... [sigh]
Wulf