I've been using Crashplan for offsite backups on my PC, Mac and my wife's Mac for the last few years. For the most part, it's been trouble free, but I've encountered a few instances of the client not actually backing up files.
The first two times, I caught it early and support urged me to reinstall and "adopt" (their lingo) the data set from the previously installed client. This worked fine on a Windows PC, but failed on one mac, forcing me to re-upload all of the data from that machine.
The last straw was the client being unable to reach the "cloud" destination for almost a month without so much as mentioning it via a notification or a simple exclamation point on the tray or anything. That's sort of a problem, no?
I found out when I accidentally deleted a folder with some not terribly important files in it, and went to recover them. They didn't exist in my backup set in Crashplan...because the client hadn't backed up this machine since the middle of July. Cool.
I had recently been in contact with Spideroak almost a year after I closed my account because they finally noticed a ticket I submitted 14 months before. The customer advocate I had chatted with urged me to give them another shot because they had improved all aspects of their service.
When I launched the newly installed "SpiderOak One" client with a new account and immediately ran into the same bug I reported in my ticket 14 months prior, I uninstalled and deleted my account. Seriously guys? The same stupid bug? Yeesh.
I briefly looked at Sync. It's a neat product, and their client and support staff are seriously nice, but it's ultimately a dropbox replacement and that doesn't suit my needs. If you want something like dropbox, be sure to check out Sync.
I found mentions of a product called Arq Backup on reddit. It seemed pretty flexible, supports a bunch of different third party storage backends, such as google cloud, AWS, Onedrive, etc.
I tested using Onedrive, AWS S3 and Google Nearline on both OSX and Windows, and it has worked really well. I don't have a huge data set, but the machine handling my NAS backup isn't exactly "modern", and it handles deduplication, compression and encryption duties just fine. It uses significantly less RAM than Crashplan with the same backup set.
Restores are painless from each service, and can be completed from any PC that shares the same backend. i.e., if you have 4 PCs on the same AWS API credentials, you can see the backups from each PC. You'll just be prompted to enter the encryption passphrase you used on each machine in order to retrieve and decrypt the stored files.
After running the trial for a few weeks, I'm pretty impressed. I think I'll be picking up licenses for my machines after I give their helpdesk a nudge about what appears to be a bug in their folder handling on Windows. Assuming their support is half as good as the software, I'll buy a couple licenses.