' difficult ' is simplistic,,, to achieve proper soil density requires MECHANICAL compaction,,, machinery ! imn-s-hfo, you haven't personally done this work OR had to guarantee results,,, think of that loose fill as a sponge !
I've been working on houses since 1973 and have yet see anyone conpact the soil when back filling.
I now live in an area where the water table can be as high as a ft. below ground. Almost no one has a basment. If someone does not back fill inside the foundation and it's lower then the outside grade I'll bet it will be fill of water in no time. We have to go back and backfill them all the time because of standing water under the house. A home inspector will not even pass a home like that here.
Most often we have to add gutters, regrade outside of the foundation and back will with what we call drive way dirt, then cover it with 6 mil. plastic.