Most of the time, after two days or so, I need to adjust the jacks because the pads/ wooden blocks sink into the ground a little. And of course they sink in unevenly.
However the main reason I use them is to spread the weight over more square inches, which makes sinking less likely.
I remember some friends with a 40' Monaco who had trouble getting their jacks to come up once on a trip with us. After cycling the leveling several times, they got the rig moved, and realized the jacks had left 6 to 8 inch holes in the ground.