Living underground allows your domicile to stay more of a constant temperature

 So, heating and air conditioning become much less of a problem once you are living deeper than 1 to 2 feet underground. Below about 2 feet underground often the ground there is around 60 degrees year around most places. But, even if it is hotter or colder than that it is pretty constant in temperature year around.

However, it's true you cannot sustainably live underground in a flood plain so this is important to consider too unless what you build is sort of like a submarine that keeps all water out whatever you do.

So, the biggest problem in living underground would be lighting and fresh air most of the year but not heating or cooling.