A config needs to be validated using a few simple rules:

1. Invalid fields are no longer supported; this helps spot typos. For example, if you type userdefaults instead of userDefaults, it will let you know rather than ignoring all of your userDefault settings you intended to set.
2. required fields, such as server.sources and different keys based on the settings that you add.
3. some additional light validation, such as min values for things like minLength for password config if provided.

You can also see the latest default config with comments here