I assume they would be "time sensitive" which is the major reason why they are withheld. Some foreign power may be able to back track from the data provided on the documents, and thus identifiy the source NSA is intercepting from.
I am confidence that many more UFO related reports exist in the NSA and the original search didn't find all the documents that could subject to the FOIA. I get the impression from the nature documents provided, indicates that the documents themselves cover a variety of subject matters, thus would be scattered in various divisions of the NSA, and not centralized in one file for example.
They maybe documents of a far higher quality than the documents provided than were admitted to, but knowing the culture and the security considerations, I doubt they will see the light of day for some time.
I know GCHQ has UFO related reports, but I also doubt whether these will be released, unless the 30 year rule applies to these files, since the UK FOIA doesn't cover GCHQ, or the security services, you cannot ask them to review them for declassification.