“1. All the Local Rabbonim agreed. (This has happened almost nowhere else.)”
Right, and all the rabbanim agreed in KGH. Actually the Minchas Asher did not agree. If Rav Moshe was so concerned with a consensus, why did he not tell the rabbanim in Flatbush that how can they make an eruv there is no consensus (see the beginning of 4:87). How could Rav Moshe have agreed that the rabbanim of Manhattan have a right to establish an eruv, when there was no consensus, and he himself did not agree (4:89).
“3. It is difficult for it to become ruined.”
Rav Moshe said the same regarding Seagate, and in fact it did get ruined.
Keep on learning the inyan maybe you will figure it out