This page lists known issues one may experience when using Seedvault to back up or restore data. If you experience an undocumented issue not listed here, make sure to raise an issue so the next person doesn't have to suffer with it.
Most developers unfortunately do not test the system backup functionality with their apps. This often leads to broken backups, backups including unnecessary data, or not including data that should be included. For a lot of apps, this is fine, they may be simple enough that they can be cloned freely and without issue, but for others, this isn't the case.
Seedvault isn't magic, it can't figure out when that's the case, so you'll get a checkmark on the status page, even if the backups don't work.
You should keep that in mind. When you find an issue with a particular app's backups not working as expected, the best thing you can do, is let the developer of that app know.