Monsey, NY has an eiruv that most Ashkenazim that live in the area hold by. I have heard that sefardim cannot hold by the eiruv in Monsey, NY. Is this true and if so why?
Rabbi Hassan describes the problem here:
Now is there a problem for Sephardim? This is a complicated area of halakha but I will try and write as short and as clear as possible.
Rav Yosef Caro (Shulkhan Arukh O.H. 345:7) wrote that the definition of a public domain is a street that is 16 amot wide (32 feet) or more that are not enclosed. Some say that if there are not 600,000 people passing through the street every day it is not considered a public domain.
There is a general rule that whenever Rav Caro writes two laws and begins one plainly and begins the other with the words “some say” we always rule according to the plain law rather than the one introduced with “some say”. However he wrote elsewhere (O.H. 303.18) that We don’t have actual halachic public domains today, all our public domains are halachically considered as a carmelit - another domain which does not have the strict laws of a public domain.
Some Sephardic halachic decisors such as the Hida and Ben Ish Hai, rule that our main roads are considered a public domain and therefore we cannot use poles and wires to make an eruv and we need proper walls to enclose the area which would be impossible to do today. However, Hacham Ovadiah rules that Sephardim can use an eruv made of wires and poles for those who need and he is the basis for my practice of using the eruv.
Hacham Ovadiah Yosef zt"l wrote (Yabia Omer O.H. 9:33) "Those that carry on Shabbat in a public domain by an eruv made with door frames (which means wires and poles), have basis to do so, and according to many authorities it is allowed even according to Maran. The sages of Jerusalem also wrote that we have no true public domain today and an eruv of doorways therefore is sufficient to carry in these days. Nevertheless he who trembles at the word of God and completely refrains from carrying will be blessed...
In contrast the Ashkenazi custom is to generally rely on those opinions that require 600,000 people to walk through the street, with some opinions (such as the Shulchan Aruch HaRav) advocating stringency when there isn't a need.
In the modern era probably every big-city eruv is relying on that 600,000 people requirement, and Sefardim have more reason to be strict about it than Ashkenazim.