I have about 600 GB in Dropbox, near 400.000 files. A couple of days ago I tested some of those:
- Maestral: Used 100% one CPU, selected only two folders to sync, only one was synchronised.
- CloudMounter: a mess with file dates, plus a lot of sync issues when uploading heavy files.
- Strongsync: Evaluated, never got a full sync.
- Mountain Duck behave similar as Maestral. I don't remember what did wrong, but failed to do some operations losing the file changes.
I think Dropbox is bad, but other those third party tools are worse when you want real hard job done. Any cloud can manage 10.000 files. Problems happens when you have it tens of thousands, even iCloud Drive.