I think the material's durability is dependent on the skill of the builder. If the cement was mixed properly and there are no voids and the reinforcing steel was properly seated and sealed so it doesn't corrode....etc. In other words, its no different than any other material when it comes to quality. Ferro seems to have gotten a bad rap because of the many backyard builders who didn't do a good job back in the day. Surveyors can look at a wooden boat and find rot, and they have been very clever in discovering ways to find hidden flaws in fiberglass boats (moisture meters, infrared imaging); maybe they have similar tricks for ferro boats.
I hear that one of the biggest problems for the prospective ferro boat owner is obtaining insurance. Make sure you nail that down before you get too invested in this boat.