We've run a fair bit of underground line at the gun club, bare. I was surprised to see it a few inches down in bed.I wouldn't do it, especially now that plastic conduit is available, but running it bare is allowed.
Hell, my TV cable is 4" deep and I can't till parts of my yard. Cable company says they will be glad to give me extra punches on my TS card. Not only that but it runs diagonally across the whole front yard, so there's not an entirely safe area.