I've been using pine for everything. McDonalds coffee stirrers are awesome for detailing. For my floors, I've simply scored the foamcore heavily and painted it to look like wood, due to material shortage. It takes a bit more time, and it's easier to screw up the final look, but if you've got a steady hand it works fine IMO.
Do you find Balsa to actually have any advantages over pine in terrain building? I can't really think of any.