These sets have barely gotten much play and no exposure at all to UV light. The brittleness is in the terrain plates and not just the water tiles, though the water tiles break into pieces and are not repairable, while the plates can be glued. It definitely seems to be an issue with the plasticisers in the components migrating with time.
I think that storage conditions and light may have accelerated the breakdown in some earlier cases, but now the breakdown is starting to happen in general with sets that are well cared for or have seen little use. I would bet that all sets will start exhibiting this breakdown within the next few years.
This makes me hesitant to buy any further Heroscape terrain, most especially any of the more expensive sets.
I think that storage conditions and light may have accelerated the breakdown in some earlier cases, but now the breakdown is starting to happen in general with sets that are well cared for or have seen little use. I would bet that all sets will start exhibiting this breakdown within the next few years.
This makes me hesitant to buy any further Heroscape terrain, most especially any of the more expensive sets.