Backing up just photo files from Photos library
You'll have your original pictures without edits, if backing up just the "Masters" folder.
From what I've seen, and depending on your configuration, this are some of the folders and educated guesses on what you will be losing:


Database folder: Database info on Albums,  faces, persons, and sharing history, keywords, places
There seems to be information on your iPhoto database, probably there if you upgraded from it to Photos.app as I did
Previews (which may be there if you have iCloud Photo Library as I do)
Projects
Faces thumbnails
Thumbnails