Garden soil has the capacity to be alive in the sense that many types of microorganisms thrive in it, and it develops a give and take with the water, keeping many chemicals in a better balance. Part of this is because of the cationic exchange capacity.
Sand is nowhere near as good at any of this, sort of a sterile nothing in the tank.
i find if i dry start the tank i dont really have much of a problem with the soil moving around if theres a little flow. never set up a tank like this but have seen it while growing plants in tupperwares and stuff. after a while it just stays together
My Mosquito Rasboras loved my cube tank when i had soil with sand cap. I have now changed the substrate to cat litter and the fish is not happy. They are hiding most of the day, they are less active when feeding and the nice colors of the fish is gone, they look pale