Are you using smart previews?
Oh, and your catalog, not the images, but the catalog. It is not on the NAS is it?
Not sure if Merge Photo works using smart previews, like most of the remainder of LR will.
Don't think I am using smart previews. (and when I grab a photo from the library it takes a while to load it from NAS - but as I mentioned it's a tolerable time)
Only images are on the NAS, catalog is on the iMac
I just did a check.
With the NAS setup, the merge operation actually did create a proper merge dng file in the NAS directory within a few minutes (and it looks like all of the exif data attached to it is correct). The merge operation then stalled (beach ball) for the next hour and a half and never completed.
I moved the pictures to a test directory on the computer and the merge completed right away.
So the problem seems to be not with creating the merged photograph but with setting up the catalog (or writing exif) for the new picture - where the picture file is located in the NAS directory and the catalog information is in the iMac directory. The thing is that in the past it somehow worked most of the time.
When I set up this NAS system about six months ago I could not find any references to anyone using is as a server. It seemed most people kept their photo files on their computer or on an attached hard disk, and only used the NAS for periodic backup. I thought it would be fun to see if I could actually use the NAS as a server and this has mostly worked although it is slower. It helped a lot when I added a direct ethernet connection from NAS to the computer (about 10 times faster than using a wireless interconnection).
Whoops, I'm actually with Lightroom 6.14
Ah, well, the v6.14 eliminates using smart previews, .