After synchronizing media that leads to a crash when thumbnailing, you cannot use the app anymore because it will never recover
Let's say you sync a file that crashes the thumbnailer of the media library, you can never use the app again. On every launch of the app, it will try to thumbnail once more and crash. There is no way to escape the loop except for connecting to iTunes and removing the file in question manually.
This is a regression from the previous MediaLibraryKit, which marked such files to never try them to thumbnail ever again.