Shipping containers can be quite good. But they can leak and be difficult to reseal, rust and be more trouble to correct than they are worth, difficult to insulate and extremely cold if uninsulated. Living for any time in shipping container can be uncomfortable unless you essentially build a room inside the steel shell.

It also has to be noted that even though they are made of steel the load bearing capacity that allows them to be stacked is all in the corners and in specifically reinforced areas of the walls. The ceilings on them are not structurally sound enough to carry a lot of weight. If you are planning on burying one underground and covering it with any significant amount of dirt you would need to reinforce the roof structure.

A shipping container placed underground in most areas will also need to be thoroughly sealed, rustproofed inside and out, and provision made to keep it from floating if the water table rises.

All that can be done, and it has been in quite a few cases, but it isn't quite as simple and easy as many people think.