Earthships are an excellent concept, and include many building styles and techniques that are useful across many sustainable building projects.
They attempt to incorporate water collection, energy collection, waste recycling, climate control, and gardens in their design. Having all these systems in one compact and efficient space is pretty cool, and can be extensively engineered to many different designs and layouts. Of course, the more integrated and compact it becomes, the more complexity is involved in the design, construction and maintenance of the structure.
Complexity aside, the building methods used often include recycling and working with earth, and are generally labor intensive rather than resource or energy intensive.
Some materials used are not very sustainable, but overall Earthships are an excellent step towards sustainability and greatly reduce carbon and resource footprints as well as utility bills, without taking up much space.