I can confirm it is completely broken.
1. Syncing issues does not occur with the files themselves, when adding or removing files.
2. I did an experiment: Rated a single file, and clicked sync preview over and over. Sometimes it would sync, sometimes it wouldn't. When it showed that it would sync, I clicked Sync, and immediately said sync failed.
2020-06-22: SM-G950U; 9; 2.0.2
21:16:30: ping: socket ok=/192.168.1.6
21:16:30: ping: hello=MusicBeeWifiSyncServer/1.0
21:16:31: ping: matched=/192.168.1.6,status=OK
21:16:31: startSync: preview=true,iteration=0
21:16:31: command: action=SYNC_START
21:16:31: tryStart: connecting /192.168.1.6, attempt=0
21:16:31: tryStart: connected
21:16:31: tryStart: hello=true,fromMB=false,custfiles=false,preview=true,dev=SM-G950U,1
21:16:31: storage: path=/storage/emulated/0,root=null
21:16:31: syncDevice: root=/storage/emulated/0,ignoreErrors=false,playlists=false,ratings=true,playcount=true
21:16:31: syncDevice: command=SendStats
21:16:31: syncDevice: command=ShowResults
21:16:31: syncDevice: command=GetFiles
21:16:31: getFiles: Get: Music/,url=/storage/emulated/0/Music/, inc=true
21:16:31: getFiles: count=200
21:16:31: syncDevice: command=GetFiles
21:16:31: getFiles: Get: Podcasts/,url=/storage/emulated/0/Podcasts/, inc=true
21:16:31: getFiles: count=0
21:16:31: syncDevice: command=ShowPreviewResults
21:16:31: syncDevice: exit
21:16:35: startSync: preview=true,iteration=0
21:16:35: command: action=SYNC_START
21:16:35: tryStart: connecting /192.168.1.6, attempt=0
21:16:35: tryStart: connected
21:16:35: tryStart: hello=true,fromMB=false,custfiles=false,preview=true,dev=SM-G950U,1
21:16:35: storage: path=/storage/emulated/0,root=null
21:16:35: syncDevice: root=/storage/emulated/0,ignoreErrors=false,playlists=false,ratings=true,playcount=true
21:16:35: syncDevice: command=SendStats
21:16:35: syncDevice: command=ShowResults
21:16:35: syncDevice: command=GetFiles
21:16:35: getFiles: Get: Music/,url=/storage/emulated/0/Music/, inc=true
21:16:35: getFiles: count=200
21:16:35: syncDevice: command=GetFiles
21:16:35: getFiles: Get: Podcasts/,url=/storage/emulated/0/Podcasts/, inc=true
21:16:35: getFiles: count=0
21:16:36: syncDevice: command=ShowPreviewResults
21:16:36: syncDevice: exit
21:16:37: startSync: preview=true,iteration=0
21:16:37: command: action=SYNC_START
21:16:37: tryStart: connecting /192.168.1.6, attempt=0
21:16:37: tryStart: connected
21:16:37: tryStart: hello=true,fromMB=false,custfiles=false,preview=true,dev=SM-G950U,1
21:16:37: storage: path=/storage/emulated/0,root=null
21:16:37: syncDevice: root=/storage/emulated/0,ignoreErrors=false,playlists=false,ratings=true,playcount=true
21:16:37: syncDevice: command=SendStats
21:16:37: syncDevice: command=ShowResults
21:16:37: syncDevice: command=GetFiles
21:16:37: getFiles: Get: Music/,url=/storage/emulated/0/Music/, inc=true
21:16:37: getFiles: count=200
21:16:37: syncDevice: command=GetFiles
21:16:37: getFiles: Get: Podcasts/,url=/storage/emulated/0/Podcasts/, inc=true
21:16:37: getFiles: count=0
21:16:37: syncDevice: command=ShowPreviewResults
21:16:37: syncDevice: exit
21:16:38: startSync: preview=true,iteration=0
21:16:38: command: action=SYNC_START
21:16:38: tryStart: connecting /192.168.1.6, attempt=0
21:16:38: tryStart: connected
21:16:38: tryStart: hello=true,fromMB=false,custfiles=false,preview=true,dev=SM-G950U,1
21:16:38: storage: path=/storage/emulated/0,root=null
21:16:38: syncDevice: root=/storage/emulated/0,ignoreErrors=false,playlists=false,ratings=true,playcount=true
21:16:38: syncDevice: command=SendStats
21:16:38: syncDevice: command=ShowResults
21:16:38: syncDevice: command=GetFiles
21:16:38: getFiles: Get: Music/,url=/storage/emulated/0/Music/, inc=true
21:16:38: getFiles: count=200
21:16:38: syncDevice: command=GetFiles
21:16:38: getFiles: Get: Podcasts/,url=/storage/emulated/0/Podcasts/, inc=true
21:16:38: getFiles: count=0
21:16:38: syncDevice: command=ShowPreviewResults
21:16:38: syncDevice: exit
21:16:39: startSync: preview=true,iteration=0
21:16:40: command: action=SYNC_START
21:16:40: tryStart: connecting /192.168.1.6, attempt=0
21:16:40: tryStart: connected
21:16:40: tryStart: hello=true,fromMB=false,custfiles=false,preview=true,dev=SM-G950U,1
21:16:40: storage: path=/storage/emulated/0,root=null
21:16:40: syncDevice: root=/storage/emulated/0,ignoreErrors=false,playlists=false,ratings=true,playcount=true
21:16:40: syncDevice: command=SendStats
21:16:40: syncDevice: command=ShowResults
21:16:40: syncDevice: command=GetFiles
21:16:40: getFiles: Get: Music/,url=/storage/emulated/0/Music/, inc=true
21:16:40: getFiles: count=200
21:16:40: syncDevice: command=GetFiles
21:16:40: getFiles: Get: Podcasts/,url=/storage/emulated/0/Podcasts/, inc=true
21:16:40: getFiles: count=0
21:16:40: syncDevice: command=ShowPreviewResults
21:16:40: syncDevice: exit
21:16:41: startSync: preview=true,iteration=0
21:16:41: command: action=SYNC_START
21:16:41: tryStart: connecting /192.168.1.6, attempt=0
21:16:41: tryStart: connected
21:16:41: tryStart: hello=true,fromMB=false,custfiles=false,preview=true,dev=SM-G950U,1
21:16:41: storage: path=/storage/emulated/0,root=null
21:16:41: syncDevice: root=/storage/emulated/0,ignoreErrors=false,playlists=false,ratings=true,playcount=true
21:16:41: syncDevice: command=SendStats
21:16:41: syncDevice: command=ShowResults
21:16:41: syncDevice: command=GetFiles
21:16:41: getFiles: Get: Music/,url=/storage/emulated/0/Music/, inc=true
21:16:41: getFiles: count=200
21:16:41: syncDevice: command=GetFiles
21:16:41: getFiles: Get: Podcasts/,url=/storage/emulated/0/Podcasts/, inc=true
21:16:41: getFiles: count=0
21:16:41: syncDevice: command=ShowPreviewResults
21:16:41: syncDevice: exit
21:16:43: startSync: preview=false,iteration=1
21:16:43: command: action=SYNC_START
21:16:43: tryStart: connecting /192.168.1.6, attempt=0
21:16:43: tryStart: connected
21:16:43: tryStart: hello=true,fromMB=false,custfiles=false,preview=false,dev=SM-G950U,1
21:16:43: storage: path=/storage/emulated/0,root=null
21:16:43: syncDevice: root=/storage/emulated/0,ignoreErrors=false,playlists=false,ratings=true,playcount=true
21:16:43: syncDevice: command=SendStats
21:16:43: syncDevice: command=ShowResults
21:16:43: syncDevice: command=Exit
21:16:43: exitSync: fails=0
21:16:43: waitScanFiles: start
21:16:43: waitScanFiles: done
21:16:43: syncDevice: exit
21:16:45: ping: socket ok=/192.168.1.6
21:16:45: ping: hello=MusicBeeWifiSyncServer/1.0
21:16:45: ping: matched=/192.168.1.6,status=OK
21:16:46: startSync: preview=true,iteration=0
21:16:46: command: action=SYNC_START
21:16:46: tryStart: connecting /192.168.1.6, attempt=0
21:16:46: tryStart: connected
21:16:46: tryStart: hello=true,fromMB=false,custfiles=false,preview=true,dev=SM-G950U,1
21:16:46: storage: path=/storage/emulated/0,root=null
21:16:46: syncDevice: root=/storage/emulated/0,ignoreErrors=false,playlists=false,ratings=true,playcount=true
21:16:46: syncDevice: command=SendStats
21:16:46: syncDevice: command=ShowResults
21:16:46: syncDevice: command=GetFiles
21:16:46: getFiles: Get: Music/,url=/storage/emulated/0/Music/, inc=true
21:16:46: getFiles: count=200
21:16:46: syncDevice: command=GetFiles
21:16:46: getFiles: Get: Podcasts/,url=/storage/emulated/0/Podcasts/, inc=true
21:16:46: getFiles: count=0
21:16:46: syncDevice: command=ShowPreviewResults
21:16:46: syncDevice: exit