They are becomming increasingly popular for dope smugglers, leaving coasties less excuse for harrasing us surface yachties. Initially, they cruised the surface, or just below, but they are evolving to cruise at far geater depths all the time.
While concrete is weak in tensile strength, it is very strong in compression, which makes it ideal for a pressure vessel , like a sub. To gain neutral bouyancy , they have to be extremely thick, not your average ferro cemnt hull.
If well built at the outset, they have almost zero deterioration in anyone's lifetime.