But in term of privacy and surveillance society, this will be a nightmare...
Imagine, the dream for all eu gov and tax/police forces to have a mandatory unified controlled system for identification anywhere. From there, it will be more easier to go one step further in surveillance for average citizen.
The thing is that currently, the multitude of not centralized, authenticated system allows to remain anonymous or quasi anonymous for a lot of things. But, once there will be an official guaranteed EU identity authentication system, everyone will want to use it.
You are visiting a city, and you need to pay the parking fee? the EU SSO will have to be used, and states will be able to know where you were, at what time.
You want to access a website that is required by law to do age verification? EU SSO has to be used and again, the state can monitor your internet browsing.
You buy/sell a few things online, on peer to peer website like "leboncoin" or "ebay"? or do car sharing on "blablacar"? Mandatory EU SSO id has to be used, and so everything you do can be monitored.