Out of all the earth sheltered ones I have seen, the owners would have preferred to go with earth beamed. Many of the opinions may have be because of the poor performance of any wood system for the roof - deflection and creep that compounded leakage because of the ponding. Goats and grass on the roof may seem to be green and cute, but there are problems.
As far as the walls a good properly built concrete or reinforced block can easily carry the loads. Both interior and exterior tiles are necessary, just as they should be used on any basement, plus use a granular backfill (not "dirt" local soil or clay). Do not tie and roof drainage into the tile drains.