In my experience the backup is very limited and most app data is lost, specially annoying are banking apps that require you to go to a physical bank branch in order to re-pair your restored phone and restore full access to your bank account.
My main issue, at least here where I live, is that you can only have a single phone paired to your bank account. If anything happens to your phone, that's it, you are locked out of accessing your bank acc. You used to be able to access your bank account from a telephone call (sure, very arctic and insecure nowadays) or from the bank website without requiring the phone app to authenticate.
Had they provided a second form of access, independently of a single phone, I would be more than fine with it.
It's not just banking apps, it's almost every app that isn't a first-party Google product. You also lose all of your files, but those can at least be backed up manually by copying them over USB.
