A bed of hairy mushrooms fills more space than leathers thus makes a more effective border. Also, neither may not exactly hurt the xenia unless the xenia gets on top of them and I think that's the reasoning behind the border strategy. In order to expand to the other side of the border, they would either have to jump (physically impossible) or go through a sea of stinging cells (in which case risk serious injury and injury may not be evident immediately. It depends on the dose of chemicals the other coral delivers and the encroaching coral's resistance to that chemical). They can get as far as the edge of these types of coral, but that's as far as they go. Though this doesn't prevent any detached polyps from being pushed over by a current, but it at least keeps them from crawling to the other side of a border.
Just for the comparison of it, I can touch all sorts of coral others generally cannot including Devil's Hand. I can play with carpet anemones as if they were velcro without any ill affects. In fact, they feel like velcro...lol. I've been speared by Christmas Urchins without any pain...though (on the flip side) have been stabbed by black urchins, which are generally considered weak, but was very painful to me.
The tolerance levels of corals seem to be a primary factor to any injury that occurs from being stung.
BTW...xenia are great for refugiums.
We, as a people, know so much more about outer space than we do about our own oceans. This lack of knowledge can very well spell the dangers that lay in wait for us.
The oceans surely would swallow us before a rock comes down to smite the planet of it's life.