Yes it is hard when all my TV Shows which were all m4v, listed under "Kind" as MPEG-4 Video File, which were converted using the same software and settings, and still iTunes will report an error it "can't edit more than one kind of media file". It doesn't happen with all, but you try to find the ones it will not work with when trying to edit the tags of 2-300 files at a time, and they are all the same "Kind". When iTunes first started throwing this error back at me the first thing I started looking at was the video format of the files, I do have both m4v as well as mp4 files, but all the files I was trying to edit were the same kind.
After looking around here I have found this is also a reported bug found in iTunes 12. These are videos I have ripped myself but there is something with them iTunes doesn't like. And it may only be one file in 2-300 that causes the issue, even though the same software was used to make them as well as add and edit the tags. The easiest fix was to find a tag editor that is not as picky as iTunes.