First, I was so relieved to find Subler. I use both MKV and MP4 for different reasons, and until now I could not do much about thumbnails, covers, and/or metadata for MP4. It is exceptional... but not as good as the sound/music metadata editors (and same for MKV) that I use... and I am speaking of actual "editors".
Unlike what is claimed on the developer's websites, this is not a metadata "editor." Importing metadata makes it a... well, metadata "importer"... not "editor." And such a shame that it is not otherwise.
I have found both TVDB and iTunes have typos, errata, and outright errors now and then in the metadata they distribute. In addition, iTunes has the worst thumbs, covers, and screens I have seen across the internet. iTunes also puts data into fields where it does not belong by media type.
Lack of actual editing combined with mislabeling the ability to do so was frustrating, because I expected actual editing capability when I downloaded Subler. But it is far better in what it does than most comparable applications in the Mac world.