I switched to Syncthing with the main desktop backed up to S3 with zbackup (encrypted). Syncthing works really well (and faster!) when keeping laptops and desktops in sync for my case.
The one killer feature I wish Syncthing had was untrusted peers like Resilio has. There has been an issue[0] open since 2014 but I guess there are still major challenges to this feature.
It would be great for me to be able to specify a certain node as encrypted and read-only like I can in Resilio. If a friend wanted to store his files on my machine and vice-versa, we could do so without being forced to give access to each others plaintext files. Likewise, if I wanted to spin up a VPS and host an untrusted node to help facilitate syncing but didn't want my unencrypted files to be sitting on the disk, I could do so easily.
Personally I was a Spideroak customer for 2 years or so and moved to Tresorit after the canary thing. They offer very similar features but their apps are a lot better. Spideroak always felt quite clunky. Also, the servers are based in Europe.
I don't think either is great to use with lots of data as upload and download seem quite slow, but for my use case of just storing documents it is fine.
SpiderOak has some serious performance issues if you have a lot of data. I just wanted to back up a few hundred GB of data but the client wouldn't do more than ~10Mbit/s on my gigabit line.
Also worth noting their "zero-knowledge" stuff is broken as soon as you log in to the website or use one of the mobile apps, at which point the server has your key.
If you want to have encryption with Dropbox, we just launched FileSafe at Standard Notes. It encrypts files client side then uploads to Dropbox (or any WebDAV server).
SpiderOak looks interesting. The biggest features for me missing from other "sync a folder to the cloud" type services are selective syncing on a machine-by-machine basis and generating public links for my files. These are the only two things keeping me away from iCloud Drive.
Their main business is personal backup, so they would be remiss to not have a good restore feature.
Each of your computers is associated with a "deleted items bin" where files go when the backup finds you delete them locally. They never get automatically deleted (even when you've reached your backup quota? the website isn't clear).