My last boat was a ferro cement ketch
A few things that I ran into -- it's easy for previous bad patches to be hidden on ferro boats. As I dug into mine I found three spots that had sustained piecing damage probably from being hit by another boat and discovered that although it looked fine on the surface that no PVA (poly vinyl acetate) glue had been used to bond the two cements AND no new wire mesh (the ferro part) had been woven into the existing structure to maintain strength.
Also, while trying to resurface the deck
(also cement) I found that it had in some places up to an inch of various coatings that had been added over the years in an attempt to cover an uneven original surface. It was a mess and in places I had found that the whole deck
(the cement itself) was totally rotten. The things that had been added to the surface had created a sort of reverse water
seal that kept the cement wet 24/7. This allowed bio material and later bugs to take up residence in the cement and finally into the plywood
onto which the original deck had been laid.
The point I'm trying to make is that although they are a time-proven material for building boats, they also hide problems really well. On the surface everything looked solid and dry. Oh yeah, and I eventually lost
the boat in a hurricane
which since I was 3 months into looking for an insurer who would touch a ferro boat, was uninsured.
Luckily I only had a few thousand into it, but had I known more about ferro boats, I would definitely still have that 3 grand in the boat fund.
Do your homework, then do some more...