-not efficient for moving at high speed
Sucks at any speed but zero. However, barges used for business or residence are mostly anchored down and left there.
-broader beam means more extremes of movement on each end of a wave
Flat bottoms give a better over all comfort when at rest. You don't get the wave action as much as the wave is distributed better but there is more side to side motion. You get used to it.
-relatively low to the water, may overwash in weather.
Barges have lock-downs on every cover. Every air intake portal or hatch would have a cover with lock-downs. The entrance to each unit would be through the top or main deck. A special hatch with the ability to open from inside or out like a submarine hatch would cover the entrance . Then if the barge does get over-washed, it doesn't matter. It's like going through a storm, kind of exciting but little danger.
Another thing is that the barge will be divided by steel walls separating each unit from the other units. This way if one unit starts taking on water, the other units will remain dry. Automated pumps to be installed in the lowest part of each unit to discharge any water before it gets to the living space just like boats have.
It is really safer to live at sea then it is to live on land. There are no tornadoes, forest fires, insect born illnesses - no insects. No earth quakes, mud slides, floods or droughts. When hurricanes hit land, they pick up trees and buildings and throw them at you, but at sea, it picks up water and throws it at you. Now that water stings like hell but it doesn't kill you. And clean up after a major storm is really easy, you just open all the hatches and let the fresh air in, the sun will take care of the rest.
I've worked the barges when I was a lot younger, both full and empty, it was really boring. We moved faster empty which caused more movement but at rest, stability was about the same as I recall.