BorgBackup (short: Borg) is a deduplicating backup program. Optionally, it supports compression and authenticated encryption, it is open source and you can self hosting it if you want.
The main goal of Borg is to provide an efficient and secure way to backup data. The data deduplication technique used makes Borg suitable for daily backups since only changes are stored. The authenticated encryption technique makes it suitable for backups to not fully trusted targets.
Borg has also a great GUI: Vorta
And they have and online storage that provides 10gb and two containers for free: https://www.borgbase.com/
All of this only work on Linux or Mac, you can set it up on Windows using a Linux Subsystem, tho.
I know this is NOT a cloud storage, I know it may not be end-user friendly but It is a great project, fully open source and everything can be encrypted so I think it should have at least an honorable mention somewhere so more ppl can at least know about it.
Kopia is still in Beta, i have tested before it looks great but Borg is more mature and have way more features.
Duplicati, Kopia and Borg do more or less the same, deduplicated backups. Duplicati lets you backup to google, mega, amazon and so on, Borg has its own “storage solution” which you can self host or you could backup to disk and use rclone for instance to move the backups to google or whatever.
So in summary, it will depend on your needs.
And I always used rsync to “sync” folders, for example I use rsync to copy my backup to my external drive, BUT, as far as I know it is not deduplicated and you don’t have “history” of what have change every time you run it. I guess here is where backintime enters, but, no, I haven’t test that combination.
Edit: And I really like Vorta over the Duplicati interface, but it is personal taste.