Shipping containers are horrible for that. They're metal so they rust and people that convert them tend to get older ones in bad shape. They don't breathe so you have to worry about ventilation and sealing the metal before even building otherwise they start to rust even quicker.

Theres basically no cost savings to using one compared to building the exact same space using stick-frame construction. Theres a reason why stick-frame is the reigning champion for affordable homes after millennia of evolution. It's really the best way (in the North American climate) to build a cheap house. Things like how comfortable, energy efficient, safe are all matters of addressing the local climate and there are solutions to all those issues.

1. Cold climates get more insulation and the plumbing is ran in interior walls to protect against freezing, the foundation goes down below the frost line.
2. Stick frame is infinitely customizable and even micro homes can be made (usually its an issue of the locality prohibiting such small housing.)
3. Live in tornado alley? We can add a tornado shelter under your garage floor if you don't want to build a full basement. Living in Florida where you can't dig a basement cause the water table is so high but you want protection from hurricane winds? Well we can build with hurricane screws which hold everything together better and even add metal storm shutters to windows.