The only cloud storage that has a decent Linux client is Dropbox.

Koofr is a decent cloud solution. Their client is horrendous across all platforms and lacks the most basic functionalities.

You can argue OneDrive / GoogleDrive are semi decent if you also use insync, which adds another license to purchase when a basic client should come free with the service you've already bought.

And no, rsync doesn't come even close to it in terms of functionality/simplicity, no matter how much hardened Linux users who haven't been outside since 1988 are trying to convince everyone otherwise.

I can recommend JottaCloud.