Should be fine as long as its ventilated and/ climate controlled in some way, if you live in a moderate temperature swing area you could be
Everything I've come across mentions that they have big swings in temperature, cold in winter but really hot in summer depending where you live. But as @Maine homestead mom said you could ventilate and climate control it for the seasons it should work well.
one other thing to be aware of is dampness/mold/mildew/mustiness. I worked at a small office years ago that used a cargo/shipping container for file storage archives. Granted they stored the files open in file crates and the cardboard file "banker" boxes, I should mention that all of the files got damaged, some more than others.
In addition to ventilation and climate control make sure seal seams and corners to prevent leaks, internal rust, etc. especially if you bury it (partially or fully).
Mold and mildew were my main concerns, which is why I said ventilation etc... Thank you for explaining my thoughts the rest of the way, I leave important stuff out a lot 😂
The steel boxes are probably not good as is. But, if you insulate it, like a house, by putting interior wall with insulation will help the huge temperature swings. Ventilation is a must.
If you are planning to store food stuff, I would be careful when buying a used container since many are used to carry goods that are not good for consumption. Food stored in these boxes may absorb the bad stuff, i.e. chemicals.
Or, to get away from that problem, it may be worth paying a bit more to get a new container.
My wife works in the ocean freight business, a freight forwarding company. She told me there are a ton of used container sitting on the American soil, since it is not worth carrying empties back on the ships. So, used ones can be had at very good prices though.