A shipping container is very strong at the corners, but the roof is not that strong, so typically you need to build another roof over it, especially where there will be snow. Also, the corrugated steel walls are important to the strength of the structure. This means anywhere you cut out a large window, or door opening requires new reinforcement. And when they are stacked together to make larger homes, welded (expensive) reinforcement is needed wherever two containers join at a spot that is not a corner. Any later renovations require significant engineering and welding.