I used Dropbox for around a decade until switching to iCloud for "better integration" around two years ago. What a debacle that turned out to be.
I have since completely given up on iCloud Drive for syncing files since it has been a nightmare ever since I started using it. On average it would completely stop working every 6 months and I would have to sign out of iCloud on all my devices and set it up again.
This last time it broke down it stopped syncing anything at all and no longer allows me to turn on Desktop and Documents syncing. That was in November and I've been going back and forth with Apple's Engineering Department ever since December and they still can't figure out why it doesn’t work.
I switched back to Dropbox around that same and have had zero sync issues. So maybe try Dropbox because while I am what you could call an Apple fanboy, I realize that not all of their products/services are very good, and indeed in the case of iCloud Drive it's a steaming pile of dog vomit.
/rant
TL:DR: I hate iCloud Drive 
-Chris