Also there's a big and increasing class of people in the EU living outside their home country. They currently have no say in their country of residence for many years and if the EU breaks apart (ie Brexit), then millions more will be left with no political rights or the prospect of ever getting them.

Not really, the only thing you can't do with a green card is vote and citizenship is automatic (modulo paperwork) after five years. That, and you can get deported for a serious crime. But the status is naturally time limited.

The second class citizens of the US are illegal immigrants. It would be trivial to get rid of them by cracking down on their employers but that would increase labor costs for their employers so the government turns a blind eye unless forced.

They are de facto but de jure they are all almost exactly the same as full citizenship.

It's a difference in degree that makes for a difference in kind. There is no legal status of "slave" or "vassal" in most (any?) common or civil law countries.