Earlier quoted context omitted.
It's complex, they can be sued or funding can get cut but EU is not like the USA. It's more like a club where the countries come together to decide how to do things and then they implement local laws to do it. They are not hostile countries brought together by force, the hostilities are usually between neighbours and EU acts as a way to resolve them. For example, countries close to the borders see large refugee influ…
If they can be sued for it, I wouldn't say it's accurate to say they can continue opposing it. Even cut funding, if they still have to contribute funds to the EU, would be coercive.
For example, let's say Poland agreed to GDPR but they went ahead and made itself a hub for data brokers with no respect to GDPR, then Poland gets sued and funds cut. Poland doesn't get sued or funds cut if they voted against GDPR and blocking it in first place.