For syncing between devices, I guess `rclone sync` should do the trick.
For example if you saved a folder of mp3 files using the rclone crypt functionality that you downloaded from a known BitTorrent, it would be easy for an adversary to match that set of known sized objects with high confidence at the storage provider end, or from observing encrypted network traffic (which in the S3 case leaks object size through traffic patterns) and deducing object sizes from there.
I totally switched off Dropbox when they limited the number of devices. Self host owncloud on a VPS, am very happy. From time to time I have to occ:upgrade something et voilà. Used for syncing, not backing up.
It helps that I can install and maintain it though, wouldn't recommend it to anyone without a bit of wed/IT experience or the time to lean some basics.
I'm not sure how much should I trust a vps host. I can mess around with encryption all day long but they can compromise my mail server without me ever knowing.
Basically, you can't. It all depends on your threat model.
edit: https://owncloud.com/features/end-to-end-encryption-2/ owncloud community and standard edition don't have e2e
The enterprise edition is out of my budget range unfortunately.
But yeah, if you have a business... don't self host too much sensitive stuff, delegate if you can.
Honestly, if you're keen to share something you shouldn't, it's pretty trivial to host it via https://rclone.org as an http server. The only person accessing it off of Google drive is the original owner and gdrive can't tell that it's being shared beyond that.