Joseph-
A more thorough answer:
I don’t remember and may never have known what all the facets of the “Brooklyn eiruv” issue are, but as best I can recall they were:
Is Ocean Parkway a rishus harabim d’oyraisa?
Do the various elevated train lines and overpasses count as breaks?
I believe many chasidim in Boro Park rely on the Boro Park eruv because that community is entirely surrounded by elevated (west end and culver) train lines, while in Flatbush they would not carry.