I actually quite like the idea of an underground house - it’d be easier to get a nice stable temperature year round and would be much easier to design for bushfire resilience (steel sheets over skylights/doorway, steel mesh over ventilation and you’re pretty set). I could do with my own personal hobbit hole.

I would however want it to be in a position where it does not require pumped drainage. That restricts you to places on a decent slope so you can use gravity to keep things dry and cozy rather than rely on pumps that could break down. Not having a view in the vast majority of locations would also be a downside, but to be fair being able to see a good bit of distance is sadly not something you get in most normal houses anyway.