Is there a way of getting a list of photos not in an album?
As of iOS 10.2.1, you can only do this in Photos for macOS.


Open File → New Smart Album.
Name your Smart Album whatever you like, eg. Uncategorized.
Simply specify the condition: Album is not Any.


Starting with Photos 2.0, Smart Albums do not refresh instantly with every change. This means that you can drag an uncategorized photo from the Smart Album into multiple folders.