To add some data to the discussion of iTunes playlists:
When I imported iTunes playlists, the import would (apparently) complete and the playlists would show up on the Libraries > Playlists screen, but when I clicked a playlist, it would try for awhile to show the song titles and then fail with a message like "Failed to load items for this playlist."
Trial and error disclosed that there is a number of songs somewhere between 726 and 1,000 beyond which all the songs are added but then this happens (Walking Blues is the last of 1,000 songs in this playlist):
Aug 26, 2016 17:32:20.287 [0x7000010f4000] DEBUG - Playlist iPhone 1-1001: Got track file:///Volumes/Glyph/Music/iTunes/iTunes%20Media/Music/Robert%20Johnson/The%20Complete%20Recordings/1-19%20Walking%20Blues.m4a
Aug 26, 2016 17:32:20.574 [0x7000010f4000] WARN - Held transaction for too long (../iTunesLibraryImporter.h:291): 2.190000 seconds
Aug 26, 2016 17:32:20.577 [0x70000019a000] WARN - Took too long (0.580000 seconds) to start a transaction on ../Statistics/StatisticsManager.h:183
Aug 26, 2016 17:32:20.577 [0x70000019a000] WARN - Transaction that was running was started on ../iTunesLibraryImporter.h:278
Aug 26, 2016 17:32:20.590 [0x7000010f4000] DEBUG - Updating metadata item (save) (iPhone 1-1001) with ID 501395The result is that the playlist is unusable.
If the playlist has less than this magic number of songs, this happens, and the playlist is usable (Born under a Bad Sign is last in a playlist of 500 songs):
file:///Volumes/Glyph/Music/iTunes/iTunes%20Media/Music/Cream/Disraeli%20Gears%20(Deluxe%20Edition)/2-17%20Born%20Under%20a%20Bad%20Sign%20%5BBBC%20live%5D.m4a
Aug 26, 2016 20:02:30.332 [0x700000081000] DEBUG - Playlist iPhone3b: Got track file:///Volumes/Glyph/Music/iTunes/iTunes%20Media/Music/Cat%20Stevens/Greatest%20Hits/09%20Father%20&%20Son.m4a
Aug 26, 2016 20:02:30.422 [0x700000081000] DEBUG - Updating metadata item (save) (iPhone3b) with ID 501407
Aug 26, 2016 20:02:30.424 [0x700000081000] DEBUG - iTunes importer: playlist 'iPhone4a' (smart: 0)
Aug 26, 2016 20:02:30.425 [0x700000081000] DEBUG - Added new metadata item (iPhone4a) with ID 501408