1. They will delete all backups of your external hard drive if you do not plug them in and fully sync them for at least ~12 hours every 30 days. You can pay an extra fee each month to have your external drives stored longer to mitigate this problem [1]. The fee is per GB so it does materially change the Backblaze pricing if you have a lot of external data.
2. Their client is extremely extremely slow, even with performance settings set to their recommendations it just obliterates CPU (irrespective of the size of the actual changeset being backed up). I've always been able to feel my computers slow way down when the Backblaze client spins up. [This issue may be resolved on Apple Silicon, but it's been a nuisance on every Intel Mac I've ever had.]
There are some other weird quirks in their macOS client, like you can set your backup schedule to "Once Per Day" overnight and it will still, multiple times per day, just start running unexpectedly and then you have to deal with the slowness or hit "Pause Backup". Somehow this happens 2–3 times every single day. I've tweaked the settings, reinstalled the client, and all of the things, but this issue has persisted for 5+ years on every machine I've used their software on. Super annoying.
Sidebar: They have a native mobile app but it is really really bad. Slow, clunky, somehow always getting logged out. The mobile client sucks so bad that I usually just wait until I'm at my desktop to do a restore.
[1]: https://www.backblaze.com/blog/backblaze-7-0-version-history....