Obviously this was a few years ago, but a backup provider failing at their one job and then blaming the customer left a really bad impression that keeps me from using them.
Backblaze eventually admitted that their dashboards aren't realtime, and they had a bug which was showing us (and their client) files that didn't exist.
Maybe it needs a kind of stochastic automated approach.. a program that finds sufficiently small (vs costs) sample of files on your computer (some old, some recently changed, etc) and tries restoring them and verifies.
At least for the standard Backblaze service you can download for free. For a USB drive you float the cost of the drive (they reimburse you when you return it)--maybe you pay shipping?
To restore it to a drive all you pay out of pocket is return shipping of the drive. The one time I had to use it I was slightly over the 30 days (I was waiting on a repair before I could restore the data) and it wasn't an issue.
I did some medium-intensity benchmarking a while back and decided not to put certain server data on it because I was getting a few 20+ second timeouts per thousand read requests. I can handle server errors, and I have retry logic, but this was something where I needed to be able to access the data within a second or two. Maybe it would have worked better if I set a very aggressive timeout, I'm not sure. Deeper testing is something I'll worry about some other time if the data actually grows past a couple hundred gigabytes.
This was mostly with the S3 API, I don't remember if I ever succeeded in getting the program to use the native one.
(The exception being the recent outages GoDaddy caused for them, but since they've moved to using Cloudflare as their registrar, I don't anticipate further issues there: https://news.ycombinator.com/item?id=26119619 )