There are a couple of disadvantages. Being the house is only 300sf per floor, there is only space for 1 primary room. This means if I am in the living room, and want to get a drink of water, I will have to go down into the basement, and get my water. Without an elevator, this could get tiring rather quickly. Likewise, due to the small size, hosting events with lots of people is not likely Some people may have to spill out to the outdoor space. This could be a problem with bad weather.
Another disadvantage is the fact that there is no garage assumed with this plan. With that in mind, if I was to have an automobile, I will have to do on-street parking. This of course can increase the chances of theft or vandalism.