I think what is slowing up adoption of this superior battery technology is
1. Lack of knowledge.
2. Shortage of suitable chargers, or at least inexpensive ones.
3. A smaller selection of capacities and sizes.
4. Higher initial cost, although over time they should be a lot less expensive than lead acid batteries which are considered to be 'cheap' batteries.
I've noticed that Norbert Bruggen has started to spec them in his models (Deepflight Aviator kit shows liFe PO4 batteries recommended). They're far less sensitive to deep discharge, are housed in tough cylindrical housings like NiMH/Nicads and have the longest cycle life of any competing technology (2000 cycles).
Lipo still retains the crown for ultimate power density, but unless you're building a very quick boat like a Subdog, I see no reason to look beyond these batteries.