I was curious why an eruv is often extremely controversial (not in terms of the secular world, I mean within the Frum world), it seems anytime a new Eruv is built there’s always a chorus rabbinic voices preaching against, at least in the 21st century. The Manhattan eruv is a great example, but I was curious why this is? The halachic concept of an Eruv seem simple enough, there has to be an enclosed area of at least 4 corners connected. So why are they so controversial?
Although there are many reasons for this I will list what I think are the main three.
An Eruv can only be made in a Karmelis (a non-biblical Reshus HaRabim) and not a Reshus HaRabim Min haTorah. Many Rishonim understand that a Reshus HaRabim Min haTorah is any street which is 16 amos wide (approx. 8m). Accordingly this would call into question many Eruvin. Even according to the opinion of Rashi in Eruvin 6b that one needs 600,000 people in the city to make it a Reshus HaRabim Min haTorah, Rav Moshe Feinstein in Igros Moshe OC 1,139 understands this that if there are 600,000 people travelling through the city not just a specific road this would make it a Reshus HaRabim Min haTorah. This would invalidate all Eruvin in cities with very large populations. There are those that rely on the Chazon Ish who says if the city is mainly fenced (Omed Meruba Al haParutz) and has no breaches which invalidate an eruv etc., one could make an Eruv with doors. However not all Poskim rely on this.
Sechirus. In order to make an Eruv, there cannot be any non-Jews living inside the Eruv area as this invalidates the Eruv. Because this rental is only a rabbinic , the requirements for rental are low. The Gemara allows a weak rental (sechirus re’u’ah) and permits rental of even just the right of a worker to place items on the gentile owner’s property. What most Eruvin do is rent permission from a worker such or police/fire department who has some rights to enter the land without the knowledge or consent from the owner. Some disagree with this with method as the police don't have a right to go into your house at anytime, there is protocol, to obtain a warrant etc so therefore that it is not considered sechirus reshus.
Upkeep of the eruv. An eruv is a vast area and some are concerned that not all parts of the eruv are thoroughly checked enough.
I think in addition that there are many different opinions of what might constitute a wall or not and how to construct it.
Since an Eruv is a Rabbinic measure there might be room to be lenient; however, we like to uphold as many opinions as possible. On your front lawn this is quite straightforward, but in a city this can be quite difficult.
Using a telephone wire as the eiruv (like many cities), well what happens when the stick that lies below it (it must be below) is crooked? It can become crooked because the telephone pole it's attached to leans or from wear and tear.
Some say it does not work and some are lenient (like Rav Aharon Kotler).
Or perhaps an Eiruv uses a body of water (ie: an intense slope or drop off). Well there is disagreement about this since it can erode etc...
When many will not use a city wide Eiruv (because it doesn't follow most opinions), that adds to the controversy. I know a Rav in Eiruvin who will pretty much never use an Eiruv except for very very few city wide Eiruvin. He hates being shown an Eiruv when he visits, because he usually sees flaws.