Syncing was... a mess

[Similan]

I don't even know where to start.

Cover images doesn't show up, or shows the wrong one (somehow default to one image in particular)
Most of the musics are by "Unknown Artist" (or credited to one artist in particular)
Half the music are missing
Most playlist are missing, and some playlist are incomplete
Overall, it looks as if the sync were incomplete, or it's only partially synced.

I'm syncing my mediamonkey musics from my desktop to my android phone. I have paid versions of both the one on desktop and android. I'm using USB (I have 5,437 files, too many for WiFi sync)
I'm follwing along the tutorial on mediamonkey's wiki page. going to "devices & services" on my mediamonkey desktop, selecting my android, going to "sync list (library -> device)" amd pick what I wish to sync (music, classical music, playlist)
All other setting are default.
I tried right click and send to -> my device, but doesn't seem to work.

I spent entire saturday trying to figure out by myself first what's going on, but came up with nothing, so I decided to try the forum. I hope someone can help me on it! It will be much appreciated! Thank you in advance.
(also English is not my first language, please ask me if you don't understand something I said)

[Lowlander]

Which Build (Help > About) of Mediamonkey are you using,

Artwork is Synced per Album, not per file. An Album is where the Album and the Album Artist values match. This also is the case when you have no value. This is likely the issue you're having with Artwork.

Wi-fi Sync works fine for that many files. Subsequent syncs would be faster as most files would already be on the device and thus wouldn't need to be synced again.

Do you get any errrors on Sync? Are you sure missing files are really not on the decice and that they were actually selected to be synced?