True, all dry wood WILL burn, but the higher density (more lbs/cu ft) woods generally provide a more consistent burn and heat output. Although Balsa will burn, if there were alternatives it would not be my first choice. I'm going to be burning a good bit of Sycamore this next heating season. It's not as dense as Oak or Hickory and so will require a bit more volume to last the season, but once its dry it will burn well and provide a good stable heat source.