Ok, I tried to backup through SeedVault.
First thing, I am using Shelter, so some apps are isolated into the work profile, and when seeing that there was a possibility to choose between personal and work profile (like in other places, allowing to have different settings for both), I thought I might be able to back up my sheltered apps as well.
Disappointment: when choosing work profile, it says “disabled by admin”. Shelter probably doesn’t support it.
Second, I thought it might have options for the backup, a bit like in OANDBackupX. But no so many actually (well, there are options to choose where and what to back up, I probably just like customization too much 
).
Third, as @Ingo stated rightfully when he tested it:
Most of the apps I would like to be backed up don’t allow a backup, but those I don’t care much about do…
Fourth, as @Ingo rightfully stated again last time:
I think it might be possible on a rooted phone through a hidden activity you can launch from e.g app manager, or from a terminal with adb shell as described in the link, but frankly it doesn’t seem easy, and as I often copy my system from my main to my backup phone, I hoped it would be easier and implemented in SeedVault, not just a workaround.
But fifth, I don’t know if the restore of a backup works well or not since the backup went to 90% quite fast, and then stayed stuck there for the last hour (probably because I have 126 apps and that’s too much and it bugs ? 
).
TL;DR: It seems really good, but I wouldn’t count too much on it for the moment, there’s still some progress to make 