I have lived and worked in 4 European countries and there are always things that you need fix even years after, for example for taxes or pensions.

I currently need to maintain digital IDs for Italy, Spain, the UK and Sweden, plus different implementations of those depending on banks/services. Add the complexity of phone numbers that are no longer valid and / or services that do not allow foreign numbers / addresses (more than you think, especially with companies).

I would LOVE to have one single ID system to rule them all (supposing it's VERY secure).