DEVONthink doesn't work for me. I have over 140.000 files on an MacMini and I can't get it to sync on my iPad.
iCloud should be working, I have there about 60Gb of files and documents, keeps a copy on all sides (cloud and all devices).
Dropbox does the same, but you need to select that you want to store locally and a year ago it wouldn't want to synchronize fast enough for my taste/amount of data.
Documents by Readdle can do really a lot, dropbox, box, icloud, yandex, FTP, SFTP, all kind of services, but also locals indeed like SMB.
It is more of a file-manager as well and the advantage is that you can copy files from one service to another (e.g. Dropbox to icloud).