Wow, with the scarcity of Organic paste in the polar/tundra regions, how are those populations supposed to build anything remotely defensible now... A small amount of metal wouldn't be too unreasonable, like 5-10 metal would be reasonable to me. Instead, now the northern groups that aren't massive are going to be stuck in wood buildings.
IMO the explosive buff was needed, however, this cost for stone needs to be immediately undone.