I've just completed a migration from one GOS phone to another using SeedVault. I'd read most of the threads about backup / restore on this forum beforehand. I had enabled data backup, and the last backup of the old phone had completed successfully. Backup on the old phone and Restore on the new one were done on the same day, using a USB stick. The restore also completed almost successfully, errors were shown for just a few apps.
However, after that I found myself restoring the settings for individual apps one by one nonetheless. In particular credentials, which are crucial for almost all important apps I use (banking apps, telecom providers, car charging, etc), had to be restored manually. Syncthing and Keypass2Android served me quite well for that, but the SeedVault restore did not. I also had to restore settings and data for OSMAnd, K9-mail, and Signal messenger manually, to name but a few. Photos, Videos etc. didn't need to be restored from backup, as I sync these to my PC anyway.
I ended up asking myself what the SeedVault based restore had been good for at all. True: I didn't have to reinstall every single App I had installed on my old phone. But that was about it.
I have some difficulty understanding the mind set behind this sort of backup / restore. It seems that 99% of the storage occupied by the backup was APKs. What's the point in using hundreds of Gigabytes of encrypted storage for that purpose? A simple text file with list of APKs would have done the same job, perhaps combined with some tool like APKCombo. (Fun fact: after installing and activating play store on the new phone, an estimated 50% of the APKs that came from the backup was updated immediately). OTOH, the user settings and data, IOW the important stuff which actually matters to me as a user, and which actually needs to be encrypted, seems to have been missing almost completely.
I realize that this isn't SeedVault's fault, as it can only do what the API provides. Still, for me the bottom line is that I can use my USB stick for something more useful than this rather pointless backup. Thus I second the advice I've seen elsewhere in this forum: for the purpose of switching phones, it's best to just restore the app settings manually one by one, using the export/import functionality provided by the individual apps. It needs to be done anyway, even if SeedVault had been used.