I wanted to provide an update on MCIS with current program version since for me this key program as I use an Iphone and others may use it too. The below may be know to many, but in case it isn't, here is one way to make MCIS work.
As posted by Imurawski (a few messages above) MCIS 6.0.2 still can be made to work with the Iphone and MC, at least with IOS 12.4.2 and MC 12.0.113. Imurawski's procedures likely work fine, but here is what I do to sync (basically, it is a start from scratch every time process instead of an updated to selected playlists)
1. Delete all playlists and songs from Itunes library
2. Update Itunes library by reimporting every song file
3. Run MCIS (first clicking rebuild for the desired playlist(s) (folder) twice - (to address an apparent old flaw in MCIS - may not be there anymore but I still do it)
4. Run Iphone "sync" with no songs selected (that will automatically be the default if you do steps 1-3) - that clears Iphone of music
5. Run Iphone sync again after selecting all playlists (or whatever you want on Iphone)
For my system, this takes a number of hours, but is basically unattended so not a lot of work.