I am slowly getting to the 2000 limit (1300) for playlists, and I have noticed the X1 will not play all the songs with random shuffle. It might play a few hundred, and then stop. Granted this is hard to test, but it is an irritating bug.
These are just local files on a direct attached hard drive. I assume they are all playable. I would expect the X1 to skip an unplayable file, and get the next file.