Off-site backups with sensitive data... I would never use a third party server to backup sensitive information unless it was 256bit AES encrypted before it left my computer with a strong key. That's my opinion and anyone can do what they want with thier own data.
nrf, how much data are you talking about, the frequency, and is this personal or company related?
If it's company related and you have another sister locations, make them your site and get the data to them somehow.
If it's personal (home), I think my comment above speaks to it well. I have a Verizon account which gives me lots of storage space. I would encrypt it and upload the data. Or you could backup your data to a USB drive and take it to work and put it in your desk. It's off-site.
Getting back to data reliability, I too can testify that I have been using Acronis to backup my home computers to the NAS and running a verification after the backups are complete which alway have passed. I used EXT3 RAID1 until last week when I made the shift to EXT2 RAID1 to obtain a huge speed boot. My specific use of the NAS is primary backup of my computers. If the NAS becomes corrupt then so long as my computer didn't die at the same time, I'm good. There is always risk, you just need to know what risk is acceptable.