I have a single sample point for sodium acetate in a hand warming pack. I came across this during our pre-move major clearout around Christmas time. I can pretty much pinpoint when it was last used, which was November 2004. I had charged it after use and put it in a drawer, where it sat at room temperature, in it's liquid phase, for over 12 years. When I initiated the phase change it worked normally, turning into a solid and releasing the same amount of heat as it always used to, as far as I can tell.
The interesting thing for me was that it had stayed in it's higher energy state for that period of time.
Looking at the basic chemistry, but not knowing the exact composition that Sunamp use, I would doubt if there are any purely age related degradation effects, as long as the seals are good on the heat battery containers.
Be interesting to find out more, but I'm leaning towards the view that the thing will outlive me.