Romania is within the EU and there is free movement of people. Meaning, they can relocate here for work. They're entitled to social security a) after living in Netherlands for 3 months and b) are legally allowed to actually live here (this latter bit is interesting). Though important to note that it appears that as soon you ask for social security they'll try to cancel your right to live here.
When Poland got the right to work elsewhere they hugely made use of that, though always went back to Poland. Leaving to situations where they do everything to save as much of their salary. E.g. living with too many people within one house, accepting way too low wages (unfair competition or sometimes illegal below CLA wages), etc. But overall it seemed there was quite a benefit.
For Romania up to now it's mostly criminals who travel. I guess due to distance (easier to drive by car from Poland).