The thing with shipping containers although very sturdy if the ground isn’t level it can often twist and pop the wood flooring up as well as make the doors difficult to open. The worst case scenario is that you build uneven then move the container finding it then levels itself damaging the work you undertook.
Now it doesn’t always have to be some complex ground as they are very robust pieces of kit you just have to make sure its level even if its soil your dropping the container onto. Also you have to look at what your needing the container for and where. For example in the UK the howling cold winds can make sitting in a container a cold place to be so sheltering it away from wind channels such as behind a wall are beneficial. But if in the Philippines for example heat is an issue so building the units on pillars is very beneficial as the air flows under the unit as well as the sun heating the ground throughout the day. This affect allows natural heating at night as heat from the ground rises during the night hours to help keep an ambient temperature. How high to place the container is then the next question and one that you need to base on the exact location you want it and needs. For example if building studio apartments maybe you want to create some reverse U shaped concrete pillars 8 – 10ft high to sit the containers on as this not only allows the heating/cooling but also a parking space to keep your vehicle in the shade when not in use. At the same time you could be just raising the same unit on pillars less than 3ft high to allow access for drainage pipes etc. which still offers some heat reflection and cooling but obviously the angle is restricted.
The beauty of the shipping container is that its physically a robust building block you can adapt to most needs as long as your practical and make sure you try to keep things as level as possible you will have little problems especially if joining multiple units. I have worked on modular housing previously and found floor levels as much as 10” out and uneven (10” over one end and 6” under the other) this has happened due to contractors involved in preparing the ground not being part of the modular construction teams not knowing the critical importance of getting things right. Instead doing a quick job for quick pay which resulted in headaches for me as we had to physically lift the units and level them manually adding an extra few days to the job. Which when your talking modular and time sensitive you have containers arriving to be joined yet nowhere to install them until you’ve fixed the first few and over a few days your starting to get problems catching up as things are all out of sequence. For the modest house builder its not too much of a problem but this was a multi storey doctors surgery built in a similar way to shipping container houses.