I’ve tried out seedvault with LineageOS and have a bit of experience with it.
First of all it’s meant to be a system app so that it doesn’t require to root the phone. That way, you can’t just install seedvault as an app, it needs to come with the OS. Which has the consequence that few people have experience with it/little information is available, as to my knowledge only Graphene OS and Calyx OS have it at the moment (for /e/ it’s in beta testing IIRC; for Lineage I had to build the ROM myself to integrate seedvault).
And to my knowledge it will not be possible to backup “everything”. App developers can specify if their app (data? not sure) can be backed up. When I tried it out some of the apps couldn’t be backed up indeed (sorry, don’t remember which).
But on the other hand, the homescreen was preserved and also several settings I checked (like volume/brightness level etc.)
My conclusion is: it’s as good as it gets for a “normal” user (i.e. when root or TWRP backup are not an option). Even if it’s not perfect.