Ferro is way too heavy for a multihull
There is no advantage to a ferro hull
other than lower initial cost of construction for home builders. In effect they save 50% of the hull
cost which is only 20% of total boat cost.
It's very difficult to insure, many surveyors won't touch it as it's difficult to determine the condition of the armature under the plaster.
Not all ferro hulls are unsafe, but many are poorly constructed and subject to electrolysis
and rust in the iron armature. This will result in the plaster cracking and chipping off, letting in more water
that causes more rust which causes more cracking until the plaster flakes off and there's nothing left to keep the air in and the water
Your boat then becomes a nice artificial reef for the fish